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Brunswick, Maine • Chaz Wing was 12 when they cornered him in the school bathroom. The students who tormented him were children, too, entering the age of pimples and cracking voices.

Eventually, he swore under oath, the boys raped him and left him bleeding, the culmination of a year of harassment. Though Chaz repeatedly told teachers and administrators about the insults and physical attacks, he didn't report being sexually assaulted until a year later, launching a long legal fight over whether his school had done enough to protect him.

Chaz's saga is more than a tale of escalating bullying. Across the U.S., thousands of students have been sexually assaulted by other students in high schools, junior highs and even elementary schools — a hidden horror educators have long been warned not to ignore.

Relying on state education records, supplemented by federal crime data, a yearlong investigation by The Associated Press uncovered roughly 17,000 official reports of sexual assaults by students from fall 2011 to spring 2015.

That figure represents the most complete tally yet of sexual assault among the nation's 50 million students in grades K-12. But it also does not fully capture the problem: Such attacks are greatly underreported, some states don't track them and those that do vary widely in how they classify and catalog sexual violence.

And with school reputations and funding at stake, there is tremendous pressure to hide such violence. Even under varying state laws, acknowledging an incident can trigger liabilities and requirements to act.

"No principal wants their school to be the rape school," said Bill Howe, a former teacher who spent 17 years overseeing Connecticut's compliance with a federal law that helps protect student victims of at-school sexual assault. "It's the courageous principal [who] does the right thing."

The attacks AP tracked ranged from rape and sodomy to forced oral sex and fondling. Assaults occurred anywhere students were left unsupervised: buses and bathrooms, hallways and locker rooms. No type of school was immune, whether it be in a wealthy suburb, inner city or farm town. And all types of children were targeted.

Children remained most vulnerable to peer-on-peer sexual assault in the privacy of a home, AP's analysis of the federal crime data showed. But schools were the No. 2 place where they were sexually violated by other children.

About 5 percent of reported attacks involved 5- and 6-year-olds. Incidents jumped between ages 10 and 11 — typically the start of middle school — and continued rising until age 14, when they began dropping as students progressed through high school.

Unwanted fondling was the most common form of assault, and about 1 in 5 of the abused kids were penetrated in some way.

The data also showed sexual assaults by peers were more common than those by teachers, which receive far more attention. For every adult-on-child sexual attack reported at school, there were seven by students.

"Schools are required to keep students safe," said Charol Shakeshaft, a Virginia Commonwealth University professor who specializes in school sexual misconduct. "It is part of their mission. It is part of their legal responsibility. It isn't happening. Why don't we know more about it, and why isn't it being stopped?"

Thirty-two states and the District of Columbia tracked student sexual assaults, AP found, though some only when incidents led to student discipline. Some of the nation's largest school districts claimed zero sexual assaults over several years, even though AP found cases in court records or local media reports.

States varied widely in whether they required any training to stop or address student-on-student sexual assault; only 18 told AP they did.

"Everyone feels like we don't have a problem, and the reason they feel that way is they have their heads in the sand," said Oregon psychologist Wilson Kenney, who has developed student-intervention programs.

The AP found that schools often mischaracterized sexual attacks as bullying, hazing or consensual behavior, and broadly interpreted privacy laws to withhold basic information from their communities.

In multiple cases, districts bungled investigations, failed to supervise students they knew were trouble, or neglected to inform parents or authorities of an assault.

Prosecutors can be reluctant to charge juveniles, and families face high legal thresholds to successfully sue school districts for not maintaining safe learning environments.

In Maine, Chaz Wing and his family battled the Brunswick school district for four years — first before the state's human rights commission, then in federal court.

From almost his first day of sixth grade in 2010, kids harassed Chaz about his weight and labeled him as "gay," according to deposition testimony and other government and school records AP reviewed. Complaining to teachers and administrators at Brunswick Junior High School did not stop the abuse, he said.

The bullying escalated to three bathroom rapes during seventh grade, according to sworn testimony by Chaz and statements he made to police, doctors, a child-abuse specialist and a state investigator. Fearing the bullies' threats, he said, he kept quiet about the sexual violence, sliding into depression and refusing to go to school.

Finally, in October 2012, after his mom found him curled up in her bed, he confided through his tears, "They hurt me."

Sexual abuse allegations can be difficult to investigate. Because many accusers initially keep quiet, physical evidence can be lost. Often, there are no eyewitnesses, leaving only the conflicting accounts of the accuser and the accused.

Brunswick school administrators said they had promptly looked into Chaz's bullying complaints — and that they immediately investigated his sex assault allegations once his mother contacted them.

The school's principal, who had no experience investigating sexual assaults, testified he found the denials of the accused boys "very credible." He concluded, after speaking with staff and surveying sites of the alleged attacks, that Chaz's claims were "very unlikely."

Police investigated but pursued no charges, despite the child-abuse evaluation that found Chaz's statements "were clear, consistent and provided idiosyncratic and sensory details." The examiner also found "strong evidence" Chaz had been sexually abused.

In 2014, the Maine Human Rights Commission ruled that the district had failed to see "the overall picture" of bullying and allowed "a hostile education environment to persist."

The state commission made no determination on the sexual assault accusations, saying the verbal and physical harassment were "pervasive" enough to grant the Wings a right to sue. Its investigator did note that Chaz's ongoing treatment for depression and a scar he said resulted from the first rape "tend to support his allegations."

After legal wrangling, the sides settled last year: Brunswick would make safety improvements, and Chaz would get $50,000 — though not the apology he wanted.

Brunswick officials declined AP's interview requests. So did parents of some of the accused students, except to say their sons were innocent. In an email, district lawyer Melissa Hewey told AP that "there is — as there should be — always an inclination to believe allegations of sexual assault at the outset. But sometimes, the evidence compels the conclusion that those allegations are false."

"The little boys who were accused," she said, "are the real victims in this case."

The AP does not usually name alleged victims of sexual assault. But Chaz, who turns 18 in a few weeks, and his parents decided to speak publicly in hopes of helping others.

"I don't want this to happen to other kids," said his mom, Amy Wing.