If you are the only male, the "Boy's Toilet" and "Boy's Changing room" will be the place girls are less likely to enter. So it's the best place to do interactions without any interruption or witnesses. But if your relationship is not high enough she will refuse to get inside So talk with her daily till you get 80% or more on asking her to follow you (some girl types can be at 70% or lower, Just try) before you take her in there.