Non-permanent tie-dye?

Hi there, I'm a senior in high school, and as summer quickly approaches, graduation does as well. See, some friends and I have been thinking about tie-dying our graduation gowns, but we're concerned as to whether the school administrators will allow us to participate in the graduation if we choose to do so. It's not that we're rebelling or anything, we just want to do something different.
Anyway, there's a pregraduation assembly shortly before the graduation itself, and we were thinking we could tie-dye our gowns for the assembly, see how the administrators react, then decide whether we'll keep them dyed for graduation. The problem is, I've been searching on the internet for the last couple of hours for a non-permanent dye, and as I suspected, my efforts have been for naught so far. If you can think of a way to dye our gowns in a non-permanent fashion, any suggestions are greatly appreciated. Thanks!