Help!

[frankenboob.livejournal.com]

I'm painting a mural on the windows at my preschool... my problem is that the green paint peels after it dries. (Why only green, I do not know.) I'm using tempera, as that's what we have on hand. Does anyone know of something I can mix with the tempera to make it stick better? Or is there some other kind of paint that I should be using (& that won't take a chisel to remove.) Thanks!