I got detention for wearing the wrong socks. So... yeah, I can imagine expulsion over a food fight (well, maybe not expulsion, but definitely suspension)
Hasn't yr 12 finished now? I suppose it must be different for boarding schools.
My school was the same though, for the majority of my time there we weren't allowed to dye our hair at all and in about my last year they changed it to we could dye our hair natural colours only.
I can understand why food fights aren't allowed though.