It was reading, writing and religious brainwashing at this top Connecticut high school.

Staffers at Avon High indoctrinated three once-happy girls into a death-obsessed religious cult that had them speaking in tongues and acting like zombies, a federal civil rights lawsuit filed by their parents charges.

Three Spanish teachers and a guidance counselor “taught students to believe in superstition, magic and a non-scientific, anti-intellectual world view,” the suit says.

“They lost their humor and their empathy. They began speaking in a bizarre new language. They became unable to think critically or independently. They became dependent on the school teachers and guidance counselor who had indoctrinated them,” the suit charges.

“All three girls experienced sudden and severe personality changes. They became flat and distant, reclusive, secretive, and non-communicative.”

The 65-page suit, which names the plaintiffs only as Jane Doe, John Doe and J.D. — says the Avon public school district and prestigious Wellesley College in Massachusetts violated the girls’ civil rights by allowing them to be brainwashed into the mysterious cult, which is not named.

Two of the girls, now 22 and 19, stopped talking to their parents and harbored fantasies of suicide and martyrdom after the teachers messed with their minds, the suit says.

A younger sister, now 16, managed to escape their clutches, it says, but not before suffering psychological damage as well.

At one point, the two older girls even freaked out when their frantic parents tried to contact them after the April 15, 2013, Boston Marathon bombing — even though the girls were near the finish line and knew their parents would be worried.

“Despite the gravity of the event, they seemed unmoved by the injuries, loss of life and enormity of what had happened, they were exhibiting little to no empathy,” the suit says, adding that the girls were obsessed with “suicidal ideation and martyrdom.”

Avon, a Hartford suburb, has the 10th best school district out of the state’s 164, according to recent rankings.

Wellesley is also named as a defendant because two of the girls went there while they were still under the influence of the defendants, identified as teachers Tanya Mastoloni, Rebecca Kessler and Christopher Esposito and guidance counselor Laura Sullivan.

At Wellesley, the older girl would perform “whirling dervishes — religious dances — until the wee hours of the morning,” the suit says.

Avon school Superintendent Gary Mala said in a statement the suit was the first time he had heard any complaints from the parents about their daughters’ treatment.

“No communications regarding that which is alleged in the complaint have been received in the past,” he told Connecticut’s NBC TV affiliate.

The suit, filed in federal court in Hartford, seeks unspecified monetary damages.