Eugene Volokh compares Hindus and homosexuals and asks why conservative Christians would (generally) be ok with Hindu school teachers but not homosexual school teachers.
I wouldn't oppose either Hindu or homosexual public school teachers (although this highlights yet another difficulty with the publicly-funded school paradigm), but I am a conservative Christian and so I can speak to what our general concerns are.
It's pretty simple really. Most parents take the sexuality of their children very seriously, and a homosexual teacher is seen as directly threatening. Not necessarily from a sexual abuse standpoint, but from a confusion and misinformation standpoint. Homosexuality isn't the only sexual issue that Christian parents have to deal with; they're also faced with teaching their children to avoid sexual promiscuity (which the world glorifies), to carefully guard their romantic emotions, and to only enter relationships with other Christians -- for starters. There are a whole host of other issues, all of which are undermined by the stereotypical homosexual. Obviously not all homosexuals are like that, but the most publically visible ones are the precise antithesis of what Christian parents want for their children. Certainly heterosexuals in our culture have similar problems, but I think you see my point.
In contrast, many Jews, Hindus, Muslims, whatever, hold many of the same social values as Christians (on the surface, at least). They're against murder and theft, and in favor of helping people and doing good. The issues that are likely to come up in a school setting that relate to religion are less threatening to parents than the issue of their child's sexuality.
Eugene specifically mentioned school teachers, but let me briefly address the larger issue of adults who work with children. I work with kids (of all ages) at my church, and one thing we're very careful about is not leaving girls alone with men, or boys alone with women. That's not because we think any of our adult workers are likely to abuse a child, but we want to stay away from any possible accusation or appearance of impropriety.
Adults who work with children are often alone with them (maybe not high school teachers, but certainly camp counselors and day-care workers). What should be the guidelines for homosexuals? Should they be prohibited from being alone with boys, girls, both, neither? The question brings the sexuality issue directly to the front, and puts it right in the kids' faces. It wouldn't be acceptable for an adult man to be the cabin counselor for a bunch of 12-year-old girls, so why should it be acceptable for an adult homosexual man to be the cabin counselor for a bunch of 12-year-old boys?
The issue comes down to publicly flamboyant sterotypes and concern over the appearance of impropriety (as well as actual impropriety, as the Catholic Church has demonstrated).