The issue isn't that the school isn't within their rights to do this. It's that they're putting an unnecessary limitation on good students, forcing them to help fight the cheaters. There's nothing inherently wrong with putting up my answers to homework on GH. The onus should be on students to not cheat; it feels kind of scummy to force students who have no desire to cheat to take down their code in a sad attempt to fight cheaters.
I guess I'm a little bit curious as to why it is so important code like this ends up on your Github account? It is pretty simple, relatively useless outside of the course, and thousands of other people are writing very similar code - I'm not sure I see the value in hosting your solution on github.
Perhaps in a perfect world, you could publicly post all of your homework and whoever is administering the course would be able to detect someone using your code for their homework - that is incredibly difficult in the best of circumstances, and in this case, effectively impossible. Doing something about low-hanging fruit - publicly available answers to the questions - seems like a reasonable mitigation step.
