You can find out who did it. Here's how.
Glass reflects, therefore when the glass was broken the act was reflected in the window. This reflection can be recovered, but first, you'll need to reassemble the window. Oh, that's step one..... reassemble the window. I know what you're thinking, but believe me when I tell you that that's the easy part.
Once the window is restored, you'll begin the process of recovering the reflection at the precise moment that the glass was broken. On the floor, draw a circle large enough to contain the window. Place the window in the circle between four lighted, white candles, one south, north east and west.
Repeat "Power of the witches rise, coarse unseen across the skies, take me back to where I'll find, what I wish in place and time." four times in a scary voice, a little louder with each iteration. Next, walk around the circle clockwise, side-stepping to keep yourself facing the circle at all times. Do this three times and then repeat it 3 times counter-clockwise.
Now this next part is really important. Look into the window and laugh (one of those crazy, diabolical ones) and say to yourself. You idiot, there's no way this is ever going to work.
Now go back to the school and ask around, because somebody knows who did it and they're just dying to tell.
Brian.