First place to look is the Billiards Education Foundation [
www.billiardeducation.org]: they have a pool in school program and they sponsor and run the junior programs that lead to the junior national 9 ball championships. Their website should have the locations and information about junior leagues and tournaments that will qualify the winners to go the the Nationals. They break out into under 19 and under 14 programs for boys and girls. The top national players go the junior worlds, which were in Reno last year. I'm not aware of any tournaments as such for high school leagues; there are junior tournaments [there was one at SBE that had over 100 kids participating.] Dragon Promotions sponsored a junior 10 ball tournament in Florida last summer; don't know if they are going to do so this year.
I live in Southern California,and believe it or not, there is no junior pool league anywhere in the area. I have a 14 year old son who plays, and we've spent the last year trying to get him to junior events when we can find them. Most of the junior league play is on the east coast, where the winters are longer and there's more indoor recreation as a result. What I've discovered over the last year i that as disorganized and confusing as the pro leagues and sanctioning bodies are, it's that much worse for juniors. BEF does a great job promoting and organizing, but they can only do as much as volunteers around the country will do for them. They ran the world juniors last year in Reno and really ran a great tournament and made the US look great.
BUT, as a general matter, organized junior pool, high school or otherwise, is really a disaster. If you've got a kid who wants to play, you really have to work hard to make the opportunities.