We hear this one quite often, especially if we come out in high school or younger. There at least is some logic behind it, because for many it is a phase. Teenage years is the time where kids change their attitudes right and left until they settle by the end; it's logical if they have had exposure to lgbt possibilities they will try on those too, but even there they settle. Time is your best friend here. The longer you have your identity the less "it's a phase" holds up.
Bide your time and go live. There's more to high school than dating and orientation status. Enjoy friends before you split paths for college.