I doubt the guy has the experience he claims he does. Usually when people say "I trained MMA for 3 years" they are bullshitting, and think watching UFC DVD's is training. many gyms do have mma classes but if he isn't taking technique-specific classes like boxing or bjj then he probably isn't taking anything at all.
Anyway 95 lbs at 5'6", yeah, that's really small. but at 17 you still have a good amount of time to grow. I gained about two inches between 18 and 24, and according to US statistics men on average gain an inch between 25 and 35 due to late growth spurts.
your mass may also gain proportionately. you can assist this by eating more, because you are thin and maybe part of it is that you aren't taking in enough healthy calories. eat high protein, plenty of carbs, don't skip the vegetables either. and don't just do muay thai, work out on your own. Learn bodyweight exercises like pushups, pullups, dips, crunches, squats, and do them every day. if you're doing it right they should become easy in a few months, and then incorporate some weights or elevations. you can combat your thinness to a large degree, height you gotta just cross your fingers.
as for this douche who wants to fight you? **** that guy. if he really "does mma" then he should understand that it's a sport, and weights matter, and it's not a tool to bully people with. if he doesn't, don't worry about him. ignore him. i know that's a tough thing to do in school when you can't control who you see and it's such an insular social situation. but remember that you have a whole life ahead of you, and no matter how things happen you probably won't even remember who this guy is in a few years. he's meaningless to your life. just a speedbump. truthfully if you're training striking you may be able to knock him out today if you got the nerve up- but what's the point? you won't gain anything by it, and in fact it could easily backfire. focus on you and attaining the goals you want, look past his petty shit. guys like that usually get what they deserve one way or the other and you'd be better off watching as it happens then being involved.
if he ever attacks you though... go straight for the balls. then the face. don't let up till he's out cold.
Everybody good, plenty of slaves for my robot colony?