The football players at Conestoga High School had a nickname for the rituals they performed each week, according to prosecutors.
Some players avoided the locker room that day, when players considered certain sexual behavior permissible.
The hazing at the elite Chester County public school, routinely ranked near the top academically in national ratings, escalated into assault one day last fall, authorities said Friday.
Three seniors, including a team captain, grabbed a freshman they didn't like, who was trying to leave the locker room during a hazing ritual. Two held him down, while the third penetrated his rectum with a broom handle, officials said.
"That's over 400 pounds of senior holding down this freshman," said Thomas P. Hogan, the Chester County district attorney, adding that the victim was 5-foot-7 and weighed about 160 pounds.
Police charged the three seniors, all 17 at the time, as juveniles with assault, unlawful restraint, making terroristic threats, and related offenses.
Hogan said the three were not charged with a sexual offense because "from our perspective, it's a physical assault and not a sex crime." He added that the decision was made after consulting with the victim and his family.
The assault took place in October, Hogan said. The victim later told his father, who reported the assault to the school last month. The victim has since left the school. Hogan said the victim does not have permanent physical damage but faces psychological effects.
The case is the second at Conestoga this school year to draw county prosecutors' attention. In November, three boys were accused of harassment for distributing sexually explicit photos of a 13-year-old classmate. One of those alleged assailants is the boy allegedly assaulted by the senior football players, but Hogan said no evidence suggested a connection.
This case is "about ignorance, violence, and a lack of supervision," Hogan said.
The Tredyffrin/Easttown School District will conduct its own "top-priority" investigation, the district said in a statement, that will look at the "level of supervision provided by the coaching staff."
Hogan said upperclassmen on the football team had held No-Gay Thursdays for at least the last three years.
Upperclassmen put their genitals on younger students' heads and ground their bodies against their legs. They also forced younger students to strip to their underwear and clean the locker room, which is what happened just before the assault, Hogan said.
They smacked their fellow students on their behinds and backs hard enough to leave marks, he said.
"There's nothing I can say right now," Conestoga football coach John Vogan said Friday, referring all questions to a school district official. The district said in a letter Friday night that he had been suspended.
Football coaches said they were unaware of the hazing, but students participating in other sports knew, Hogan said. "This was the worst-kept secret in the world," he said.
Navin Zachariah, 18, the sports editor at the student newspaper in 2014-15, said he did not believe hazing had been going on for three years or more, as Hogan said. He also said he was shocked by the assault allegation.
Zachariah, who graduated and is now a freshman at Pennsylvania State University, was good friends with last year's leaders of the football team.
"They would not have allowed this to happen as captains," he said. "I know in my heart that's true."
Will Klein, 16, said he had heard students say casually, "Oh, it's No-Gay Thursday," but did not know about physical abuse. "I thought it was a joke," he said. "I didn't think it was a real thing."
Many social-media commenters said the students should be tried as adults.
But Hogan said the victim's family approved of the decision to charge the three as juveniles. If the family wanted them charged as adults, "we would have taken that into consideration," he said.
Two of the students were processed on Thursday. The third is out of state and will be processed Monday, Hogan said.
Conestoga, in Berwyn, is routinely ranked among the top public schools in the state for academics.
About a month ago, school officials said police were investigating reports of hazing.
Hazing allegations prompted cancellation of football seasons in 2014 at Central Bucks High School West and Sayreville War Memorial High School in Middlesex County, N.J.
Police did not file criminal charges in Bucks County. Officials did file sexual assault and hazing charges in the Middlesex County case.
Staff writers Justine McDaniel and Rick O'Brien contributed to this article.