A drop-in class resets every week. You teach to whoever walked in, which means you teach as though nobody in the room has ever met you, which means week fifty-two is structurally the same class as week one.
You have probably felt this. It is why experienced teachers get bored, and it is why students who love your class still plateau in it.
A six week series is the smallest possible fix. Same skill, same hour, same room, one change: the same people, all six weeks, signed up in advance. It is the shortest step between teaching classes and running something of your own, and it is the one most teachers skip on their way to trying to organize a retreat.
What a closed room lets you do
The first week feels similar. The difference starts in week two and it is enormous.
You can build. Week two can assume week one, which means you can teach something that takes more than an hour to learn. Nobody has to be re-oriented, nobody arrives mid-arc, and you stop spending the first ten minutes of every class making the room safe for a stranger who may not exist.
You can give homework. Five minutes a day between sessions, and then week three opens with "so how did it go", which is a conversation you have never once had in a drop-in room.
You can be harder on them, in the good sense. In an open class you teach to the most cautious plausible body, because someone might have a shoulder you do not know about. In a closed room you know about the shoulder. By week three you know all of them, and you can stop hedging.
And there is the thing nobody warns you about, which is that the students form a group. They start talking to each other in week two, sitting near the same people in week four, and by week six a handful of them have swapped numbers. That group is worth more to your teaching life than the money, because it is a group that comes back for the next thing.
None of this requires more skill than you already have. It requires the same people twice.
Six weeks, not four, not ten
Six is the number for a reason, and it is mostly a booking reason.
Four weeks is too short to feel like a change. People do not notice a difference in four sessions, so they do not have the experience that makes them sign up for the next one.
Ten weeks asks a person to promise ten specific Wednesday evenings, and no one can see ten weeks ahead. The ask gets refused, or worse, it gets accepted and then unravels in week seven when the group thins out and the energy goes.
Six weeks is roughly the length of a school half term. It is short enough that people can look at a calendar and say yes, long enough for somebody to actually feel their hamstrings change. If you want longer, run two sixes with a gap, and let people re-book. The re-booking is a feature, not friction: it tells you honestly whether the thing was good.
Also pick the boring slot. Same weekday, same time, same room, all six. Every variation you introduce costs you attendance, and the variations feel small to you and large to a person with children.
Pick a question, not a theme
Series that fill have a specific promise. Series that do not fill are called things like "Deepen Your Practice: A Six Week Exploration."
The test is whether a person can tell, from the title, whether it is for them. "Backbends and the fear of falling backward" passes. "Six weeks for people who sit at a desk all day and whose neck has opinions about it" passes. "Learning to breathe for people who have been told to breathe and have no idea what that means" passes.
Specific does not mean small. A narrow promise brings more people than a broad one, every time, because a broad one gives nobody a reason to decide.
Pick the thing you are actually interested in this year. Six weeks is long enough that teaching something you are lukewarm about will show by week three.
The numbers, honestly
Price it as a course, not as six classes.
The usual shape is that the whole series costs somewhere around five to five and a half times a single drop-in class in your city. So the student gets roughly one session free against drop-in prices, which is enough to make the decision easy and not so much that you have devalued your own hour. Some teachers price it at full sixfold and it still fills, particularly when the promise is specific.
Do not offer a per-week option. The moment a series can be paid weekly it is a drop-in class again, and every reason for doing it disappears.
For numbers in the room, work out your break-even first. Take the room cost for six evenings, add whatever you spend on making it exist, and divide by the series price. In most cities that lands somewhere around four to six people, which is a much smaller number than teachers expect and worth calculating before you start worrying.
Then set two thresholds and say them out loud to yourself before you open bookings. The minimum you will run it at, and the maximum you will take. A closed series of eight is genuinely better teaching than a closed series of twenty, because the whole point is that you know everybody's shoulder.
And decide now what you will do if you get three. Running it anyway at three is a legitimate choice, as long as it is a choice: you are paying for six evenings of practice at teaching your own material to a small room, which is not nothing. Cancelling is also legitimate. What is not legitimate is running it at three while feeling like a failure for six weeks, which the students will feel too.
If you can, take payment when they book rather than on the first evening. People who have paid come in week four when it is raining. People who intended to pay do not.
It fills from the room you already have
Almost nobody fills a first series from strangers. You fill it from your Tuesday class, and it takes about three weeks of mentioning it.
Say it once at the start of class, in a normal voice, and then have somewhere to send them. The link matters more than the announcement, because the person deciding is standing in a doorway with wet hair and will not remember four separate facts by tomorrow.
"Quick thing: I am running six weeks on backbends, Wednesdays from the 14th, same time as this. Small group, same people every week, which is why I can do things I cannot do here. It is on my page if you want the details."
Then say it again the following week, and the week after, because roughly half the room was not there the first time. Teachers announce things once and conclude nobody wanted it.
Ask two or three specific people directly, before you announce anything. Not a group message: an actual sentence to a person you have watched for months who you know it is right for. Three people who have already said yes make an announcement to a room feel like an invitation to something real rather than a request for help filling it.
When someone misses week three
They will. Somebody always misses week three, and how you handle it determines whether they come back in week four or quietly disappear for the rest of the series.
Here is what happens if you do nothing. They feel behind. They assume everyone else learned the important thing while they were away. Week four now requires walking into a room where they are the person who does not know, so they skip it too, and by week five they are gone. You will read this as a lack of commitment. It is almost always embarrassment.
So the fix is to remove the shame, fast and casually, on the day:
"Missed you tonight. We did the twisting version, nothing you cannot pick up. Come five minutes early on Wednesday and I will show you in two minutes. See you then."
Three things in one message: I noticed, you are not behind, here is exactly what to do.
Build for it too. Open every session with three minutes of what happened last week, which serves the person who missed and quietly re-teaches everyone else. Do not offer a refund or a make-up class for a missed week, and say so at the start, kindly and once. A series where the sessions are individually redeemable is not a series.
Week six is where the next thing starts
Do not let the last session end with everyone drifting out.
In week five, tell them what is next: another series, a workshop, the fact that you teach one-to-one, whatever is true. In week six, say it again and give them a way to act on it that evening, while they are standing in a room with people they now know, feeling the difference six weeks made.
That is the actual mechanism. Not a bigger room, not a better poster. Six weeks with the same twelve people, ending on the evening they can feel it worked.