A believe there is case law that suggests the government can take action to ban ACTIVITIES of a certain religion as long as it is not targeted at the religion itself (is the Peyote/Indian case still good law? If not, use bigamy prohibitions instead).
If the goals of Satanism are to thwart authority (I think I just used a “loaded” word there…), then the school could legally ban any group whose goals were to undermine or encourage the breaking of school policy or state law. IF the Satanists fall into that category, then couldn’t they be removed from campus? Narrowly tailored I am sure is the standard here.
But, on the matter of the Christian kids being rude to non-Christian groups as a justification to not allow religious groups in the schools, I offer this: peer pressure does not equal government action. There is no legal mechanism in the world to prevent high schoolers from being jerks.