what is the biggest lawsuit filed and won for religion in a public school?

Answer 1

Probably - in terms of effect, if not monetarily (no money was awarded) - the "Lambs Chapel" case.

In "Lambs Chapel" the Supreme Court ruled that in allowing the use of school facilities by non-school groups, religious groups must be treated exactly the same as other groups.

The school in question allowed clubs to rent the meeting rooms at the school when school was out. A prayer group, "Lambs Chapel", wished to rent the hall but the school said no, they weren't allowed to rent to religious groups. The case was appealed to the Supreme court, who ruled that a school must treat rental applications from a prayer group EXACTLY the same as one from, say, a chess club or music group.

Richard