Open source it and upload it to Github. There's no rule against giving away your code, is there? It sounds as though there was no prior agreement whatsoever.
Assuming it's OK to open source it, let her know you're going to do that because you own the code by virtue of having created it. If she gets nasty, tough. What's she going to do, sue you for open sourcing your homework assignment? That will just make you famous (and her, infamous).
If you haven't signed an IP agreement or employment contract, I think it's pretty obvious that you own the property with at most an unlimited license given to the school.
Assuming it's OK to open source it, let her know you're going to do that because you own the code by virtue of having created it. If she gets nasty, tough. What's she going to do, sue you for open sourcing your homework assignment? That will just make you famous (and her, infamous).