Don't worry about it. I generally blame them for this, but in your case, it's going to be good: a great proportion of guys just don't care, don't ask and don't wonder about things. "Blagh, it's just like that... dude." Since you'll have a binder (right?) it might be best to change in a private area (there's always at least one toilet in a locker room, is there?) so that you don't raise eyebrows... In that case, you might get picked on a bit for being shy/prude, but it shouldn't last so much.
And even if you didn't, I don't think they'd care much. If they ask, it's an undershirt, 'cause your skin is sensitive to clothes rubbing on it. That will be the end of the story.
I was picked on by three people at high school, who did me one mean trick (lolol, let's move his clothes whilst he washes in the private shower so that he has to come out - I actually never came out and screamed for a teacher), just once, and it stopped.
Although I'm MTF, I can share some experience that might be useful to you. In a few aspects (notably voice and some parts of the face) I pass pretty well, which gave me quite a few passes from strangers even before HRT (I was cheating a lot though by not wearing -exclusively- male stuff and being in a coat and scarf~). My voice doesn't really sound male, at least to the foreign ear, and I don't really act male. Yet I haven't really been bullied for such things... Except maybe once or twice in high school because that one jerk had noticed that I was shaving my arms and legs, but it stopped almost immediately.
And in college, where I'm pretty much presenting as androgynous, the only one comment I got was from that almost-friend (^^') guy who asked why my hair was long. Oh yeah, and two rude strangers discussing: "that's a guy? I thought it was a girl" - "no that's definitely a guy" - "ah I guess", etc.
Some of the guys avoided me ("I'll catch the gay!!" or stuff? Even though I'm pretty much asexual at the moment) for a few months, but then they realised that I was a decent and sometimes helpful person.
And all that whilst looking pretty much like a "person of ambiguous gender", not even trying to look all that female; just doing what I can to cope until full-time. If you actually make efforts to look male, how could it go wrong if you say you pass fairly well already?
Just have confidence and go for it! It's most probably going to be entirely fine.
Aaaaaaand even if they find out... So what? You think they'll... beat you? Christ, if they do that for such a trivial reason, they're gonna beat people up daily. Plus I think people are a little more accepting of FTMs than MTFs, due to the fact that genetic men tend to be considered perverts by default, for what it's worth.
Nono, seriously, even in super unlucky scenarios, I don't see it going "that" wrong.
And don't forget this: the more you think and worry about misfortune, the more likely you make it to happen. I don't have any logic to back this up, but naive and carefree people tend to get in so much less trouble than worriers, don't they? Regardless of transsexualism or anything, actually, when meanies are going to tease someone, they target the person who they think will feel it the worst, who looks like they're going to be anxious about the consequences of denouncing, who looks like they're going to be afraid of them.
Act the contrary of that and implement into your mind the idea that worst-case scenarios are less likely than sudden death by meteorite fall (okay I'm exaggerating a little, but~) and you'll be adding to your arsenal an extra shield against the already low likelyhood of such events.
