I recall that about 10 or 15 years ago, the Supreme Court ruled that if extra-curricular clubs and local civic organizations were allowed to use a school's facilities, then religious organizations must also be allowed use of the facilities in a non-discriminatory manner.
I don't know if the ACLU was directly involved in that case, but here is the stand of the
Illinois ACLU:
If a school allows any kind of non-curricular club to meet on school grounds, then it must allow a religious club to use the facilities as well. Teachers and administrative officials must stay out of club activities, except for monitoring for acceptable behavior of all students.
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