I tip at restaurants, bars my barber, and tattoo artists. There might be a few other situations on a case by case basis but if I order standing up from a till, that's not grounds for a tip.
"I can't drop out, but this is too much. Won't the other parents, I don't know. What would they say? Besides staying over with a teacher? Won't that get you into trouble with the school?" I ask seeing if there is any wiggle room