How
I don't know if this is the only way but this is how I did it
== SPOILERS ==
After playing soccer with them for a while I decided to just hang out with them instead from end of school (2 PM?) right until the end (5 PM). When they ended the game they invited me to hang around with them behind the old locker room. This is the key. Afterwards it's possible to have sex with them in several different ways when hanging out with them after the game.