You have to get to the second floor balcony in the main courtyard, then enter the room opposite the big stairs. After a lot of beds and a small open area you'll end up in a room with a black hole in the ceiling, that's the bell tower. Climb the ladder.
Oh, man, getting to that area was just about my biggest facepalm moment ever in all of my Fallout adventures. I came out onto that walkway around the courtyard before the bunk room, turned right and ran into the broken edge of the walkway, well out of jump range to the bunk room.
I spent at least another hour of time frantically looking everywhere for some other way to reach that room, driving myself utterly crazy in the process, before finally noticing that if I had turned left on that walkway instead of right, I would have noticed that it led all the way around to the bunk room, completely unbroken.
:facepalm: :facepalm: :facepalm: