Sidewalk Chalk - Legality?

Along with what the first answer said, writing on the sidewalk could be considered vandalism. This depends on the local laws, and the rules of the school. Also, depending on local laws, hate speech could elevate the severity of the crime. Even if it is legal, you could face disciplinary action at school. Lots of colleges have rules that go beyond the law (such as dry campuses that don't allow alcohol on campus). Just because it is funded in part by the government doesn't mean it's a free-for-all.