Is community service mandatory for high school graduation at a private school? Can diploma be denied because of not having community service hours?
It depends on the school and, if they require community service to graduate, then they can deny graduation. Their school, their rules.
I agree with Mr. Wagner. Private schools have more authority to make up their own rules than public schools do. A private school's authority is generally defined in its contract with students' parents, though such contracts often refer to other documents as well. Many private schools require community service, and there is no law which forbids them to do so.