I have taught many classses, not in right angle weave itself but lots of others. Twelve in a group is lot of people in my opinion, but if you have a skilled helper or if you have scheduled enough time it can be done. If your students have never done this technique, I would suggest starting with something simple. I have a sample of what the finished project will be so it can be referred to during class and also I provide instructions for the piece being made so they have something to take home and refer to again. If the project is small or extemely simple do only then would I include another project. Tending to a large group can take a lot of time in itself. I ususally limit my classes to 6 people. If they are not bringing their own supplies, I would make project kits ahead of time for everyone, this also assures that everyone has the correct materials. I usually only give two color choices to keep that aspect of preparation simple. I think you should attempt to make a project while you are teaching them, especially for the beginning of the class to help get them started, sometimes you do get to work on it f they catch on quickly, but you should be available to them throughout the class if they need help. You might consider having an extra project almost to the completed stage so you can help them finish up when they are ready.
I hope this is of some help to you and good luck, teaching is a lot of fun!
SueW