Removing Poster Paints from Glass?

I work at a movie theater, and a month or two ago, after I was noticed doodling by one of my bosses, I was asked to do some window painting for Surf’s Up, which we were promoting.

So I got to work, and in about a week or two, I had painted some surfing penguins on the glass front doors and windows of the theater. I did the “lineart” for them sparingly with Sharpie, and then used cheap craft poster paints.

So now that we’re about to lose Surf’s Up (to make room for Harry Potter), the time is approaching to disassemble all the promotional stuff for it, including my removing the painting.

My question is, what’s the best way to go about it? I’ve tried using the simple spray cleaner stuff we use there, but it doesn’t do that much good. It WOULD work, if I stayed there scrubbing and scraping all day. I’m sure there’s got to be some better way to get this stuff off.