Schools may take steps to protect those trusted to the care of children from speech that can reasonably be regarded as encouraging illegal drug use without fear of violating a student's First Amendment rights.
The phrase could be viewed as celebrating drug use—bong hits are a good thing,” or “we take bong hits”. Again, the school did not want someone to misunderstand and think the school promoted illegal drug use.
The First Amendment declares that Congress will not make laws respecting an establishment of religion or prohibiting the free exercise of religion. The Fourteenth Amendment says that states are not competent enough to make laws regarding this issue.