American Civil Liberties Union

From the ACLU.org website

Religion In The Public Schools:
A Joint Statement Of Current Law

The
Constitution permits much private religious activity in and about the
public schools. Unfortunately, this aspect of constitutional law is not
as well known as it should be. Some say that the Supreme Court has
declared the public schools "religion-free zones" or that the law is so
murky that school officials cannot know what is legally permissible.
The former claim is simply wrong. And as to the latter, while there are
some difficult issues, much has been settled. It is also unfortunately
true that public school officials, due to their busy schedules, may not
be as fully aware of this body of law as they could be. As a result, in
some school districts some of these rights are not being observed.