I’m not interested in defending this teacher necessarily, but yes, it could have been an honest mistake, particularly if the student didn’t have a chance to explain why there were ashes on her forehead (you know how some teachers can be - no backtalk!) I have seen a Protestant friend come up to a Catholic friend on Ash Wednesday and say “Omigosh, you have a HUGE smudge on your face - hold still, I’ll get that for you!” entirely innocently.
On the other hand, if the teacher was aware of Catholic practice on that particular day, then that incident was no different than than if the student had been wearing a Muslim headscarf (hijab) or a Hindu bindi and had been forced to remove those, and equally reprehensible.