At every junior high school and high school in Greater Phoenix's East Valley (Mesa, Tempe, Chandler, Gilbert) there is a Mormon seminary adjacent to each. This has been the general practice for years.
My old junior high is an urban island, a property to itself surrounded by paved streets and houses. Somehow, the Mormon church there carved out their own privately owned acreage. If I didn't already know better, I'd say the Mormon seminary on the campus is owned and operated by the public school system.
As far as I know the Mormons don't use this as a prosletyzing center. At least, I was never invited to the building (not that I would have attended). They have an agreement with the public schools here to allow for an hour each day for Mormon students to attend class in their buildings, typically I believe before or after a regular school day. I've never seen a Catholic seminary or a Baptist seminary -- or any other seminary -- built. I suppose, legally, the school systems would be required to oblige. Perhaps no other religious organization has thought of this. I doubt many churches would have the public means. Most churches are more focused on taking their kids out of public schools for private schools or home school. Mormons are the only organization I know of who are wholly supportive of public schools.
I'm curious if this is a common practice outside of the American southwest. Obviously, Utah practices this, and the East Valley was originally founded by Mormons, so the church's influence here is much greater than I would imagine it would be in other areas. I'd be interested in hearing from folks he have either witnessed this in other parts of the country, and especially if they are just now seeing this as new schools are built.