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.
Homophobia is high in middle school, but by high school it seems to fade, particularly amonst seniors. surveys show that more than 80% of high school seniors support same sex marriage.
*
30
a bunch
54.3%
43%
like 30 to50%
Around about 1 third of High school students but it also depends on the type of school and where it also located.
My guess would be 92%
12.3%
Less than 2%
zero ... magna cum laude is an honor to designate college graduates, not high school
70 %