Paula Prosper worried that her son was not ready for the differences between his private Montessori school and the public Fairfax County seventh grade she planned to transfer him to next year.

Prosper, a teacher, asked if he and she could sit for a few hours at Longfellow Middle School “to see what happens in classes and to get a feel for the school in general.” The answer was no, with explanations that made little sense.

Prosper said Longfellow’s director of student services, Gail Bigio, told her “it had to do with privacy issues for the teachers — the public employees whose salaries are paid by my tax dollars. Then she brought up immunization and likened it to the students attending the school who wish to have a visiting cousin shadow them.” Longfellow principal Carole Kihm told me Bigio did not mention teacher privacy.

There was a big reaction when I wrote recently about a similar case — an Arlington County couple who asked to observe a class for an hour to see if the popular Arlington Traditional School would be right for their children. They also got a flat no. Having them sit quietly in class would be too disruptive, Arlington officials said, without making clear why.

This touched a nerve. That column got three times as many comments as usual. Readers were deeply split. Some shared my view that the policy left parents in the dark for no good reason. Others thought the ban on observations for prospective parents prevented potentially harmful intrusions.

“You could have current parents, prospective parents, graduate school professors, school system evaluators, reporters, as well as non-custodial parents and pedophiles parading as prospective parents entering schools to observe classes,” said a teacher signing in as dcpsinsider. A reader using the name scrapper1952 said: “I can’t believe out of all of the things wrong with public education, that keeping a few helicopter parents happy is worth bashing a very good school district.”

On the other side was a parent signed on as LaborLawyer. He said he used to spend a day each year following his elementary and middle school children through all their classes and learned much about teacher work loads and classroom conduct. An educator signing in as JDunning said: “It’s long past time that we welcome parents in to see what we do and what they think of it instead of hiding in the world’s largest cubicles.”

The resistance to parent observations is not so much a policy as an unexamined taboo. I received e-mails from educators I know and respect asking why I was spending time on such trivia. To them the observations were obviously inappropriate, yet they admitted they knew of no cases in which parents watching classes ever created a disturbance.

Some urban charter schools that have succeeded in raising student achievement and improving life skills use the arrival of visitors as a lesson in manners and public relations. Students take turns greeting the strangers, making eye contact and briefing them quickly and quietly on the day’s lesson as they sit down to watch.

The Fairfax ban on lengthy observations turns out not to be based on teacher privacy or disease fears. Schools spokesman Paul Regnier said: “We provide extensive information on our schools and the learning environment, curriculum, special programs, and more,” but “extensive classroom visits by prospective parents can be disruptive to the learning process” and “our schools simply do not have the staff to accommodate prolonged visits.”

Kihm, the Longfellow principal, had just one example of trouble from a visiting student observing at another school. The student fell and needed stitches.

Banning observations won’t stop students from falling down. Why are such fine districts reluctant to let interested and involved parents watch the great work they do?