First lesson: middle schoolers (think 11 - 14 years old) will test your patience in new and completely unimagined ways, if you don't have kids of your own. They will also surprise you in a bunch of wonderful ways too. I had to learn how to corral their attention, navigate their relationships with each other, and motivate them to actually get stuff done. Turns out it was great training for being a supervisor later on :)
Second lesson: I had to learn about flight, more than I'd ever before, and figure out how to teach it. I don't think I did this very well, to be honest, but five years after graduating school it was nice to teach myself something that wasn't tech-related.
Final lesson: I had to learn to accept there were things I could not fix. As an engineer and inveterate tinkerer, this is hard for me to come to terms with. Even in the affluent area I lived in, and with kids who had all volunteered for this after-school activity, we had kids with serious home issues, kids whose friends were dealing with hard personal problems, and at least one violent outburst. I saw the kids for ninety minutes a week; the best I could do was make a welcoming and safe environment for them. That has never really sat well with me.
My job changed, and now I travel too much to coach anymore. It's my biggest regret about the change. I miss working with those kids.