The students also say they are reluctant to confront sexist behaviour by a small number of cadets and athletes, underscoring the need for commanders to improve the climate at the academies, according to a Pentagon report.
Students believe their leaders take sexual assault seriously but "also identified peer pressure as a barrier to reporting," Major General Jeffrey Snow, director of the Sexual Assault Prevention and Response Office said yesterday.
"That's not good," Snow said.
"Sexual assault is a crime and has no place at the academies, just as it has no place in our own forces," Snow told a news conference.
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Sexual assaults decreased at the US Military Academy and the US Air Force Academy in the 2012-13 academic year, compared to the previous period, according to the report.
But the number increased slightly at the US Naval Academy, rising from 13 cases in 2011-12 to 15 in 2012-13.
The Defence Department needed to instill a climate of dignity at the academies where students "possess the social courage to take action when faced with situations at risk for sexual assault, sexual harassment, and inappropriate behaviour of any kind," Snow said.
The report, which included focus group surveys of students and faculty, said cadets and midshipmen admitted "reluctance to report incidents of either sexual assault or sexual harassment due to perceived damage to their reputations."
"They believe that calling such attention to themselves can diminish their social standing among peers and have a future impact on their careers," it said.
Officials said it was crucial to counter sexism and harassment, as they fuelled further sexual assaults.