For Tony Hawk's Pro Skaters school 2's pool level, you must do a few things before you can gain access. In Tony Hawk's Pro Skater, there are always hidden buildings or places that can be accessed across the city and they are always really fun to skate in once you get in.
To unlock the school 2 pool level do the following:
Tony Hawk Pro Skater School 2 Pool: How to Access it
Firstly, you must turn right down the nearby alleyway and then once at the end of that, make a left down a pathway towards the end. Follow that pathway until you come across purple and green lockers.
At that point, a timer should pop up towards the center part of your screen. When the timer strikes 1:42 you will hear a bell go off and see a golf cart driving in front of you. By jumping and then grinding on the rail in front of you, go to where the cart came from.
Once you have done that the doors for the Matt Helsom Memorial Gymnasium should be open to you, all you have to do is skate through them. Then when you are inside, a big empty pool will be to your left in which you can preform some crazy tricks.