In mccollum v. board of education, why did the supreme court strike down a public school "released time" program?

The answer is the religious classes were held in public facilities.

It was struck down because what the schools were doing was unconstitutional as it was breaching the first amendment. They were funded by tax-payers and they made people study religious things, and the supreme court deemed that this was unconstitutional and that religion should be kept outside of schools.