Lol one, teens do have lots, lots and lots of sex.
I do agree with your point that the BF is only straight for the main guy, unless you put background on him, I only like to do those kinds of stories unless the POV is in the 'straights' bf point of view, so it makes more since, because if he just pops out of nowhere then I'm like 'wtf?'
With the diversty, I guess it just depends on your imagination and the story itself. For example I'm okay with the rich, popular guy saving the poor, bullied kid, it's part of the story line and with the rest I think it's just part of the cliches with the cliques and such, if you see a lot of those, it's because almost everyone of our American schools have those.