What percentage of high school students are homophobic?

Everyone and nobody can be classified as "homophobic" because there is no precise limit for that Word.

If we understand "homophobic" as discriminating gay people, then everyone who doesn't want equal rights is homophobic, because this is a discrimination (i'm speaking objectively).

I'm in France, so we are not in the same country, but i think the opinion poll about gay marriage might be a good indicator of the homophobic persons in your country.

There is no opinion poll all over the world, that's why you have to check your own newspapers, and no precise answer to your question.