I'm told that only exclusive behavior counts towards the 10% figure, so frequency shouldn't matter - if it's exclusive, you count, whether it's once or 100 times. As for tuples, do you mean in the sense of having relations with more than one person at the same time? Seems kind of adventurous for high school, but maybe I led a sheltered life back then ;)
Anyway, I guess you could record it as two separate acts if you encountered such a thing...