The pool is available from the start of chapter 2. Are you in chapter 2 yet?
After you can punish the girls yourself there is a cut scene on the Monday morning and it says chapter 2 in big letters. If you're in chapter 2 and you can't unlock the pool then all I can say is you've made a mistake downloading it or updating it. Just download the latest version. Unpack it into a new folder and play. Saves will transfer automatically.