Saw this movie when I was a kid on TV, it was about this teenage boy whose like in a troubled youth school or something and he starts falling for this girl who is either in the same school or in a nearby all girls school can't remember, I know they weren't allowed to be together though so near the end they runaway in to the woods and there being chased by the authorities cause there trying to runaway together and in the end they jump into the water and go to a underwater cave that he found before or had a map for, sorry so vague think it was in the 90s

Fire with Fire (1986)
Starring Virginia Madsen, Craig Sheffer
Wikipedia synopsis:
In this fact-based adolescent melodrama, Joe Fisk is a juvenile delinquent who falls in love with Lisa Taylor, a beautiful Catholic girls school student, in an Oregon forest. The two meet by accident when the troubled young man stumbles upon her while being chased by his peers in a training exercise and sees the lovely girl floating in a small lake as she works on a photography assignment, recreating the Pre-Raphaelite painting "Ophelia" by John Everett Millais. The two are immediately drawn to each other, but neither of their custodians encourages contact with the opposite sex, and, when their relationship is discovered, there is trouble all around, forcing the young lovers to flee. The question remains: Will they be able to escape the law and other authorities long enough to find happiness?

http://en.wikipedia.org/wiki/Fire_with_Fire_(1986_film)