A school isn't the place for religious indoctrination, whether it's done by the teachers or the pupils, the principal was in the right.
If she was asked not to, warned, then did it anyway then she has no leg to stand on. If she wasnt warned then she has a case for freedom of expression.
That's a damn lie, you take that back! Anyway, if the student had said something about how christ had been a big part of her success I'm sure that would have been fine. But what she was doing there was obviously preaching, so go go gadget principal.