I'm talking about guys who medically transitioned before/during high school.

I'm post op and been on T over a year. I'm unquestionably male at this point.

When I transitioned, I was already out of school on homebound (thats where they send you a tutor and you do your work with them 2 hours aday). I had only spent about 2 months in school that year (10th), and only the first half of the year before(9th). Last year (11th) I went to an alternative school where no one knew me and lived stealth. I didn't really have many friends at my old school and i didnt really keep in touch with anyone after I left. This year I'm going back to my old school. I'm going to just act like I'm the new kid, but I know it will get out eventually. Probably about 50 or so people there know I'm trans but its a small school and people are going to find out. I'm going to be using the mens bathroom and locker rooms. I have no idea what's going to happen about that when people find out.

If anyone has transitioned before or in  high school, could you please tell me about your experiences? Especially regarding locker rooms.

i started T summer before 9th, and had top surgery summer before 10th. i'm going into 11th now.
i am stealth except for principal, counselor and nurse. i go to school with 2 ppl who knew me as female. one doesnt say anything and the other told my math teacher (her aunt) who was fine and kept the secret.
I do JROTC instead of PE and pre op just changed in stalls, post op I change in stalls/ open bathroom.unless you're on swim, there's really no reason to change underwear. I've changed with and without a packer and even in my binder in front of tons of guys, no one cares or looks.
if you do PE and have required showers, that'll be an issue that needs to be discussed with school admin.

I was in 11th grade last year. 

I wasn't and still am not on T but was able to take a contracted PE course where I recorded my physical activity and showed up for the final exam, already in my gym uniform, on the last day of school.  My school offers the contracted PE for those with no room in their schedule or those who are shipped off to the Vo-Tech building every day.  I didn't fall into either category, but when I discussed issues unique to trans kids in PE (uniforms, locker rooms, different grading systems for males vs. females) with the school social worker, she was unexpectedly understanding and put me in the course.  If I had wanted to take normal PE, I would have had to change in the nurse's office, which is, for some reason, all the way across the school from the gym, and walk over in my clothes, and I felt that would have attracted much more unwanted attention to myself.

You could see if contracted PE is an option in your school.  I would highly recommend it, especially since it meant I was exempt from at least 50 miles of running, countless push-ups, and beration from gym teachers. 

I took JROTC  in 10th grade before I came out and medically transitioned, so I really had/have no need for PE. However, it's an easy class, so I took it last year (11th grade). I'm also taking it this year (12th) because it's an easy elective lol.

We had 2 teachers because of some layoffs. The first one everything went smooth, pretty much no guys used the locker rooms, we all just wore our shorts and shirts under our uniforms and changed in the gym.

My 2nd teacher was a total jerk and made us use the locker rooms then said I couldn't use the male locker room because I'm legally female. I didn't bother fighting him about it (legally I have the right to use it, but I wanted to avoid potential drama). I just used the mens bathroom in the front of the school. But then security had an issue with that which was the beginning of A LOT OF ISSUES with staff . .  but I'm helping the principal and the President of city council in my city to develop some policies to make our school district more prepared and aware of trans issues.

I'm not sure what's going to happen this year (12th grade), but I plan on using the mens bathroom and locker room since I (1.) have the legal right, (2.) am comfortable with it and (3.) well....I just want to use the bathroom, jeez.

Don't let my issues discourage you though! Some people just need to be educated and that's what needs to be done at my school. It's just a process, you know? I suggest letting your principal know in case anything happens. Best of luck!

I started to transition at 7-9 grade, where I dressed male and binded my cheast, but my life where horrible,
I didnt had a real binders, I couldnt get my parrents to buy me one as they wasnt suportive, and I couldnt shop outside my country.

neither was I allowed to me male in school so it was just my friend who knew, but they couldnt use male pronouce about me. generally I wasnt accepted as trans for the teachers, so I guess I only was out for my friends.
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I moved school at 10 grade (its normal here) and got rejejcted first for being trans but got in later,
a couple of people knew I was trans, my teacher where pretty friendly and just accepted me as male, together with the classmates with no doubts or problems. I had got my new name and started on homones in the middle of the year.

the next school I was on, I also had problems getting in for being trans, but got there anyway, I had to get out to be trans the first week and it was sort of messy, I had to use a seperated locker for showering (not toilet we got unisex)
after second sememster I didnt need to come out since homones had infected so much that I already passed I started to use the male shower as some of my friends said nobody would care, and there where curtains so you couldnt see anything, I did that and also started using the male swimming locker after I got top surgery.

my current school/work, is messy, im not out as trans exept my teacher who got my ID number.
he accept me and only use male pronouces, the rest of the folks dont know, im very unsure since one guy seams alittle transphobic, so im not sure if I should come out or not. its the first time I been living stealth, but I think I will get out sooner or later cause I dont really like being stealth to be honest..