Same advice as everyone else really, use your head and be careful. My own favorite time to hit schools is over Spring Break, Christmas break, Thanksgiving, and just after school lets out for the summer. Apart from holidays, Sunday morning seems to be the best for me. What I usually do is start off "detecting the corners", which is kind of hard to define but it's more or less starting off somewhere neither in full view of all traffic around the school nor lurking in the shadows. I don't act nervous, but I don't look at and wave to every passing car, either. I look like I'm working, not like I'm waiting for the cops to come see what I'm up to. I guess the best advice would be that if you don't feel comfortable where you're searching, something's wrong and you should probably leave. I have left public parks where it just didn't feel right, usually because single young adults were meeting up with other singles or groups of two for just a few minutes at a time. Maybe drug deals, maybe not, but not where I wanted to be regardless. I'll be the first to admit having carried a firearm to a few of the places I've detected (I have a concealed carry permit) but NEVER anywhere NEAR a school. Aside from being just stupid, it's a felony.
If you do find a high school you're comfortable with, don't limit yourself to just high schools. You might not think of pre-teenies as having enough loot to lose, but one of my all-time favorite places to hit are the playgrounds on elementary and middle schools. A lot of playgrounds have wood chips scattered around the bases, and these serve multiple purposes such as preventing scrapes and scratches on the kiddies, making a hard landing off the swings not so hard, immediately hiding coins that come out of pockets, and keeping those coins from getting buried in dirt. They come out like new, all you have to do is get your beep on and move some wood chips with your foot.
Actually just about every square inch of ground on a middle school is fertile soil for coin-shooting. Obviously around any picnic tables or decks is where I hit first, any playground equipment, a 20-foot circle around trees, then 6-8 feet to either side of any sidewalk. If you don't have any luck with these places, skip to the next school on your list because someone has already beat you out. I'm currently working on a "conglomeration" of schools in a neighboring community. The high school, middle school and 2 buildings of elementary are all on the same 2-block patch of ground. The high school is the worst to find stuff, there's no place for kids to hang out between classes or when school lets out. However there's a courtyard (grass and dirt) between the middle school and elementary schools, with two different areas of playground equipment, and yesterday I pulled out 172 coins. Last week it was 153. Got my second Sacajawea dollar yesterday, and pulled a 1943d Mercury dime out of the dirt in that courtyard. No reason for it to be there, but I'm not going to wonder about it too much.
Schools are great, they get re-seeded every year with coins and you can go back again and again as long as you observe the basic rules of safety, fill your holes and don't cause a scene.
Good luck!