I don't know of a study that has quantified the "most" allegation. But what I have heard from relatives who teach (or resigned from teaching) in public schools is that a large enough number of parents take this approach and seem to come to the parent-teacher conference with the "my great kid is entitled to grade X" attitude... that teachers' authority/professionalism gets undermined.

Even worse when administrative does not have your (teacher's) back.

If you doubt that the Peter Principle still applies, spend some time in faculty meetings and check out the admins. There are exceptions, of course. But IMHO (observation) the "No more A-holes" rule is more honored in the breach than the observance.