I've found that it can vary between people. There was a "couple" when I was in school who were seeing one another from about the first year until the fifth, and it all ended in tears when the boyfriend cheated on the girl with one of her friends.
She ended it when she found out, but when he realised what an idiotic mistake he had made, he tried and tried to get back into her good books, but she merely took advantage of his stupidity and used him for lifts at 2am after partying with her other friends, for which he was willing.
I saw this unfold from afar, being that I was not a member of their popular clan, but it was abundantly clear that he, being a teenager was over lusty, and when the possibility presented itself for the chance at another girl, he leapt onto it (and her).
That being said, they did manage an on and off relationship for over five years, and it may have even stemmed into lower sixth, I'm not sure.
Other than that, I witnessed (and experienced) a lot of relationships that started in school and died a quick death in the duration, leaving the people involved bitter and resentful in the aftermath of brutal break ups. One of my ex boyfriends had the whole option of another girl (total slag at that), and he too was a lusty teenager and decided to betray my trust - and it was just a vicious circle for everyone in the school.
Once again, I can contradict myself by saying that this is not always the case - my neighbours have been together since high school, and they have been married for about 15 years (although given I hear them arguing almost all the time through two very thick walls).
So in short, I think it depends on both persons' willingness to stick with it, be faithful and love one another for as long as they can, without breaking any hearts or deception.
