"High school" in the US is for ages 14-18, give or take. So before college/university/whatever you call it. And the reuinions are usually basically privately organized, so no one's looking for a handout. Not to mention that most people go to state-run schools (called "public schools" here), so there's probably not any way to donate money to them anyway.

It's really all a social occasion. It's just that most of us have no desire to be social with people that hated us ten years ago.