Are they vehemently opposed to you transitioning, or do they just not really understand it? If it is the latter, perhaps you could try seeing if they would take you to see a therapist that specializes in transgender issues? Or show them videos of other trans-guys so they realize it is something possible? Documentary-type things? If it is the former, well then you might just have to wait it out. I had to do that also because my mom wouldn't let me transition.
As far as your school situation... Well, I guess if you wanted to tell your parents about it, that could be done after you got them on your side with the methods described above. If they find out and aren't happy about it, there isn't much you can do formally, but you could politely ask each teacher to refer to you with male pronouns/etc. And if there is some sort of LGBT club at your school, you could get their support.
Hopefully that helps a bit? Also, welcome to Susan's, Jeffrey!
