A high school team can practice on school grounds, and nearby parks, and compete at local orienteering club events. Some high schools send teams to compete at regional and national championships.
Links:
Orienteering Coach's Handbook (Cascade Orienteering Club)
NJROTC Orienteering Training Manual (NJROTC Area Six Leadership Academy)
QOC Library (Quantico Orienteering Club - available for QOC members to check out for up to 3 months)
Orienteering Training (Orienteering USA)