DULUTH — The former Proctor, Minn., high school football player charged with sexually assaulting a teammate last fall was placed on probation Monday and must register as a sex offender for 10 years.

The 18-year-old, who was 17 at the time of the September incident, pleaded guilty last month after an agreement was reached between the defense and St. Louis County prosecution to keep his case within juvenile jurisdiction, rather than asking the court to treat him as an adult. On Monday, Judge Dale O. Harris placed him on supervised probation until his 21st birthday. If he violates probation, guidelines call for a four-year prison sentence.

The Star Tribune typically does not name juveniles charged with crimes.

The teen was charged by juvenile court petition in January with third-degree criminal sexual conduct — a felony — for assaulting a teammate with a plunger.

The victim's mother read a statement from her son Monday. He said the incident, led by someone he was close to, "will affect my life forever." He and his family have endured rumors, pain and stress, and he is receiving therapy, he wrote.

"I have to think about it every day," the statement read. However, "I feel like [the charged teen] realized what he did was wrong."

The charged teen, accompanied by his parents, spoke in court, apologizing to the victim and his family.

"I made a mistake I will forever regret," he said. "I ask that you will still look at me as a young man and believe change can happen."

Harris said he received 21 pages of character letters for the charged teen, and commended the victim's family for the compassion they've shown him.

Addressing the perpetrator, Harris noted the teen had earlier said the assault was a joke that had gone too far. Harris asked him rhetorically, "What level of humiliation and degradation is acceptable to you?"

He also said he was confident that he hadn't acted alone, and while some won't face punishment, some may. Harris noted there was a football coach who either created or permitted a "toxic culture," and school employees who knew this and failed to intervene.

"It's clear to me the problem went deeper than one action on one day," Harris said, and that he hopes the teen's young age gives him a chance to change his ways of thinking.

The investigation began in mid-September after a complaint of "student misconduct" within the football team. It led to the resignation of coach Derek Parendo and the cancellation of the football season. Proctor police completed their five-week investigation in late October.

According to police, football players revealed that a plunger was regularly used during "crass" locker-room behaviors, such as touching other players with the rubber end. The charged teen in May called the September incident "a joke that went too far."

After practice Sept. 7, the petition says, the victim was in the football locker room when he was confronted by the suspect with the plunger. He fled outside toward the field, thinking it was a joke, and was chased by the suspect and other players.

During questioning from attorneys and Harris in May, the teen named six other players and their roles in the assault, including chasing, tackling, holding down the victim and pulling down his shorts. On Monday, county prosecutor Korey Horn said he couldn't comment on potential charges against any of the six named boys because of their ages.

The teen sentenced Monday must also complete 80 hours of community service and attend counseling.

The Proctor school district of about 1,800 faced intense scrutiny in the wake of the incident. Student athletes became the subjects of taunts and jeering from opposing teams and fans at athletic events, and many Proctor students were afraid to wear clothing that identified their school. Others, upset with silence from school leaders in the wake of another school-related sexual assault allegation, questioned whether the schools were safe.

Last fall, Parendo told the Star Tribune the allegations didn't coincide with the culture of discipline and accountability he promoted among his players.

"We talk all the time about how you are part of Proctor football," he said. "People don't know your name; you are part of a group. You do things for others, not yourself."