Alright, I noticed you restarted the game, hopefully the second time you managed to avoid the bug. To everyone else that may find themselves with this same problem, since the devs so graciously provide us with a Debug menu I was able to find out what the problem is. You need to have seen the "music club is now able to play together well" scene in order to progress Ameli's questline. However, if you have "Nude uniform" dress code rule going, the game won't ever play that scene. Presumably because the scene features clothed characters? Who knows why the devs decided to block that scene in this case. So go back to your PC, get the students to wear something, see the scene, progress Ameli's questline until she asks for the Cabaret just to make sure (I don't feel like checking the other events to see if they won't work properly if students are nude) then you can get your students to wear their birthday clothes without having any problems with Ameli's quest.