Hi Laurent,
Thank you for reporting this! This is some sort of bug; I've encountered it myself when I was trying to make my video walkthrough.
I'm not quite sure how or why it's occurring, and I sat down tonight to try to replicate the bug, but couldn't successfully do it; tonight it was working fine.
When you stand in front of the one girl, Latoya, it IS supposed to automatically trigger the rest of the quest. But for some reason, Abigail (the shy little girl) gets stuck.
My hunch is that it might have something to do with your player followers. When I initially programmed this quest, I turned off the player followers (Carmen, and Candace if you added her too) and it's possible that Abigail can't walk through them, even though they're transparent.
If you haven't tried it already, try walking the LONG way around the playground slide, instead of moving down and hitting that spot directly in front of Latoya. You might have more success that way.
Till I can figure out how to replicate this again and identify a better way around it, I would try reloading the game, or if possible, restarting from an earlier safe point?
If that doesn't work, don't worry. Marquis, who is one of the football players (he wants to join the dance team), has a bugged quest where you can get MULTIPLE hearts after completing it. So if you can't get Abigail's heart, talk to Marquis twice, and he will give you one.
I'm so sorry about this bug!