I use the guys locker room at school, I had some problems, but I called some legal groups and got it straightened out.
I'm going to do this in steps
Step OneAs Jason said, it's a couple of questions first. .
1.) Do you present and pass as male?
2.) Are you out at school or stealth and are the other guys comfortable with that?
The first question is for sort of obvious reasons and the second kinda piggy-backs it. By piggy back I mean, if you present and pass as male and the other guys are comfortable with it then cool, move on to step 2. If you don't pass as male, and/or if the other guys aren't comfortable with it (assuming you're not stealth) then there's going to be a problem. They'd probably spazz out and draw unwanted attention and all of that with your mom that you don't want to happen will probably happen =/.
Step TwoAssuming you live in America . . check here to see if your city/state covers you as far as public accommodations
http://www.transgenderlaw.org/ndlaws/ngltftlpichart.pdf . If they do and you pass/the other guys are comfortable/cool with you or you're stealth (you already said you're not) then you're good to go. If your teacher says anything, have a copy of that printed out and tell him that you are legally allowed to use that locker room, etc.
* If your city/state does not protect you legally, then you pretty much will have to talk to your principal and hopefully she'll let you use the guy's locker room. Or just go and use it and hope no one spazzes or makes a big deal.
*If you don't live in the US, then look up laws in your country.
Really, if you're transitioning in high school you NEED your parent's support simply because people are jerks and will try to give you problems with simple stuff like using the locker room. Or if you have some type of LGBT group in your school, talk to them and see if they can help you out. Otherwise, I hate to tell you . . you're going to have a hell of a hard time without some type of support.
It's really messed up the stuff we have to go through, but that's the reality of being trans for the most part. . =/