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Thanks! Must be an update, because from the same videos, they enter the girl's bathroom long before entering the boys bathroom :/

How do you get in the girls bathoorm? I've seen videos of people getting inside but it's locked for me. I'm missing one page.

Its on hole 4. Follow the course until you see a bunch of large rocks by the shore. You have to jump on the smaller rocks until you reach the furthest rock. The pearl is on the other end of the last rock. 
Of course you need to unlock jumping by getting the hole in one achievement