For me, this is a "camel's nose inside the tent" issue.
For about 50 years, the religious right has been all atwitter over the banishment of prayer from our public schools. There are very good reasons for the ban, which need not be detailed here. While permitting religious groups to rent out school facilities for after hours proselytizing sounds fairly innocuous on its face, I suspect that, at bottom, this is actually an ecclesiastical flanking maneuver aimed at getting around the ban.
We no longer live in frontier communities in which the one public building must serve as weekday town hall and courthouse, Sunday church, and nighttime saloon and bordello. The faithful very often have their own buildings, called churches or synagogues or mosques, for promoting their own beleifs. A congregation that cannot afford to build, purchase or lease such a facility, but is sufficiently flush to rent out school premises, is evidently capable of paying for use of the local VFW hall or Elk's Club for an afternoon or two a week. Failing that, let their members donate use of a basement or living room for the purpose, much as cub scout den mothers used to do in the dim, dark days of Ozzie & Harriet.
Now, I'm sure the faithful will point out that there are conveniences inherent in the use of school buildings (e.g., the presence of a captive audience, avoidance of the burdens attendant to moving the kids, etc.), not afforded by the other alternatives I have suggested. Well, too bad. There are burdens attendant to most human endeavors. Just as obstruction of religious practice is not a legitimate end of government, neither is its facilitation. The rule is simple in both concept and application. Controversy over it can really only exist in the minds of those bent on employing the coercive power of the state to spread their own religious doctrines (and there is at least a subtly coercive aspect to the use of public school property to advertise and provide religious instruction).
Not being an absolutist on most issues, however, I would offer the faithful this bargain: in exchange for use of public school facilities for religious marketing, allow government to use church vestries immediately after services for seminars on evolution and contraception. Think there will be many takers?