Go looking for the girls dorm.

You peek out of your room to make sure no teachers are around, then walk down the hall. You pass a guy who is a similar age to you coming out his room.
"Hey man, where's the girls dorm?"
"Down the hall, turn left, through the double doors."
"Thanks."
"No problem."
You follow his directions and arrive in the similarly laid out hallway. You see a girl at the other end of the hall, phase through the door into her room. There's also a sign that points to the girl's shower room.