I was at school round about the time that Rangers had their 9 in a row and all that crap. The school was FULL of Rangers fans who acted like utter tools to be quite honest. I remember one of them brought in a fanzine once and quite frankly, I wouldn't have wiped my ar*se with it.
The few Celtic fans there were, were generally polite and courteous in comparison, and I don't believe it was self preservation either. That said, I didn't go to a Catholic School (I'm not a Catholic anyway), but I have been told the difference between a Rangers fan and a Celtic fan a few times since and, I'm afraid, if I had to choose, I'd be a Celtic fan any day, the lesser off two evils and all that.
Of course, there will always be the lunatic fringe on both sides, I'm not denying that, but generally, I have always found Rangers fans to be a lot more obnoxious about things.