I used to work for GW and did this sort of thing. I would recommend you keep it to something like 3 students at the most per person teaching, more than this becomes alot to handle depending on ages and enthusiasm. Each lesson will take longer than you think it will people who aren't used to handling brushes have no idea how much paint they should have on them and will often under load meaning they go back 5-6 times for a small area or they'll overload and flood it. I would teach only basecoats on the first go if you are setting aside 15-20mins for each.
Things I would focus on teaching are loading the brush/not overloading the brush and thin coats/coverage. Don't forget to have a model and brush for yourself to demonstrate on. I would stick to 3-4 colours at most if possible.
I would try to avoid teaching anyone younger than 11-12.
These are obviously generalisations some young kids will be fine and focused, some will be far too enthusiastic and will thus take much longer for you to teach as you constantly have to get them back on task, same goes for adults
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It's been a while since I worked there but those are the things that jump to mind if you have any other specific questions let me know.