their expulsion from the school for words they used that amounted to a violation of the school's code of conduct isn't a 1st amendment violation, i think, b/c the students aren't being deprived of any right by a government actor. if the state school had jailed them for saying 'nigger' then that'd be different. if the state school had seized any of the students' property that'd be different. if the school had fined them it'd be different.

all they lost is a privilege which was granted to them upon their agreement to abide by the school's code of conduct - which i assume details the various punishments to be levied by the school for violations, up to and including expulsion. the school has found they violated that code and so the students are expelled under the agreement between the students and the school.