Would anyone know of a video contest for high school kids?
No sorry, best best is to setup a GOOGLE ALERT for something like Film makers competition or Film contest. That might give you a head start on ones that pop up.
Check out this website here; http://productioncrate.com/?_ga=1.260015562.1180294189.1487679672 they will be having contest every month the one for this month is almost up.
Hope this is what you were looking for?