One of the ghosts or apparitions who attacks Dawn and the other two kids in the school is a janitor and I am wondering if that is the same janitor who was killed in the episode: "I Only Have Eyes For You" from season 2? If you recall, in that episode, several "couples" become possessed by the spirits of a couple who died at the school in the 1950's and they play out the events of the incident. In that incident, a male student killed the teacher he was dating when she broke up with him; and then killed himself. One of those twosomes who became possessed was comprised of a teacher and a janitor. The teacher and janitor were not actually involved but when the spirits possessed them they had an argument as if they were breaking up and the janitor shot the teacher and then he shot himself; just as the couple from the fifties did.
None of the other "apparitions" from the season seven episode appear to be familiar but the janitor might have been intended to be the same character from season two.