“The reason why the middle school stinks is that there are so many terrible teachers,” my friend told me about her son’s Upper East Side establishment.

“Does the head of the school know that everyone thinks the teachers stink?” I asked.

“I know plenty of people who’ve complained. But unless you have very deep pockets, not much gets done,” she said.

Educators agree that teachers play a critical role in predicting academic success. Some experts in the field, such as a senior research fellow at the University of North Carolina, William Sanders, suggest that teacher effectiveness outweighs all other factors as a predictor of student academic growth.

If that’s the case why is it that schools — particularly private city schools — continue to employ mediocre teachers? Private schools don’t have to tackle cumbersome unions or challenge the delicate tenure process: Independent school teachers have year-to-year contracts, and there’s hardly a shortage of qualified teachers — private schools receive hundreds of applications when they offer a teaching position.

“Parents are always complaining about the teachers,” a board member at an independent school in Riverdale said. “There is always one teacher that’s better than the others and parents get peeved when their child isn’t in the right class. Of course some teachers are better than others, but parents have to look at a child’s education over the course of many years. There will be exceptional teachers, good teachers, and hopefully very, very few inept teachers.”

Some picky parents should remind themselves that even with top-dollar tuition, nothing is perfect. Still, why don’t schools fire inadequate teachers?

Although private schools can hire and fire as they like, and often do, schools have to be careful to avoid costly lawsuits, according to some teachers and administrators.

“Administrators have to have a good reason to fire someone and very often will manipulate things to do it,” an administrator at a school on the Upper West Side said. “For example, they’ll hire outside evaluators to come in and assess a department of a school and these people will essentially be paid to ‘recommend’ a restructuring of the faculty.”

An independent school teacher who works on the Upper East Side told me that sometimes all it takes is a little grease. “There are times when a school will offer a teacher a financial incentive to leave, sometimes as a way to butter them up, get rid of them, and make sure they don’t sue for being fired,” she said.

This teacher explained that sometimes schools base their decisions on who to hire and fire not just on the teacher’s ability in the classroom. “There are some teachers — often highly qualified ones on paper — who have Ph.D.s or are artists who show in galleries and museums, who get away with murder because the school likes to cite their qualifications for public relations,” she said. “These teachers are often the least interested in teaching. Some are angry that they’re not in a university. This is, of course, a generalization,” she added.

Not surprisingly, parents who have deep pockets are often able to produce the greatest change. “For years, people complained about this useless high school math teacher,” a mother of three children who attend a private school in Brooklyn said. “Nothing happened and I swear, hundreds of kids graduated this school not having a clue about algebra because of this teacher. And then one year this teacher taught the child of the school’s biggest donor. The parents freaked out and the teacher was fired the next year.”

Well-connected parents can use their influence in the other direction, too. “Even the worst teacher is likely to have some parent advocates, some of whom have deep pockets,” the independent school teacher said.

There is the possibility that the teacher with the terrible reputation isn’t so terrible after all.

“We kept hearing about this terrible English teacher,” a father of two teenagers who attend school in Riverdale said. “My daughter was horrified when she found out that she was in her class.”

“But the teacher is wonderful,” he continued. “She has pushed my daughter and forced her to work harder than ever. Some parents want their children to have teachers who just tell them how smart they are. Some parents feel that if they’ve forked over $25,000 in tuition, their children shouldn’t have to work that hard or ever get a bad grade. Some of these kids treat the teachers like they work for them. Frankly, I don’t know how the teachers do it.”

sarasberman@aol.com