Schools should do everything they can to stop bullying. This has a huge impact on the lives of children, since some feel they don't have the skills to defend themselves. Demeaning jokes is at the heart of most conflicts, and if they can be avoided, these bullying encounters can also be avoided.
Public schools (here, I will assume that it is a K-12 school and not a university) are traditionally zones where students give up some rights in order to receive a free education. Students are not allowed to dress in any way they please, they cannot bring firearms or other explosives to school, and many behaviors that are legal are still not tolerated at schools. As such, demeaning jokes are just an extension of these forfeited rights.
This sounds to me like infringing on someone else's free speech. People, especially men, use demeaning jokes all the time as a way of bonding. It is just simple banter that is not bullying. How then would the university be able to differentiate between a joke said in jest or one where there was malice behind it. It would waste far too much time.
Public schools have so much more important things to worry about. There is already an unwritten rule of zero tolerance when it comes to demeaning language or jokes anyway. If a child gets caught saying demeaning things they are disciplined. Now most kids are smart enough to say things out of the earshot of adults, so it's almost impossible to enforce this rule but they know better.
I feel that banning this type of joke is just going too far towards infringing the right to free speech. It would cost a lot of resources to constantly review and determine whether what one person said was indeed demeaning. I don't think that some behaviors can be eradicated by setting up rules like this. It would be futile.