Oh, so YOU were the jerk that didn't help me fix my bike. If you thought I was drunk last time, wait until I show up for your next class!

On a more serious note, do you charge money for these classes? I find that people are more invested (literally) in something if they have to fork over money for it. If it's free they are less likely to take it seriously or even show up. Five or ten bucks per person should do it, paid in advance.