Students were subject to restraint and seclusion hundreds of thousands of times in one school year, a new analysis finds, and that’s likely just the tip of the iceberg.

During the 2011-2012 school year, there were more than 267,000 instances of restraint and seclusion reported by the nation’s schools. The figure is believed to be low, however.

The numbers come from an analysis by the nonprofit news organization ProPublica, which crunched data collected by the U.S. Department of Education’s Office of Civil Rights.

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Among students who were restrained, three-quarters had disabilities, federal officials said when they first released the data in March. Such children also represented 58 percent of those placed in seclusion rooms or some other form of involuntary confinement at school, the agency indicated.

Though the Education Department sought to gather data on the use of restraint and seclusion in every public school across the country, only a third of districts reported using the techniques even once, ProPublica found.

What’s more, schools in New York, Chicago and Los Angeles — the country’s three largest districts — reported no instances, leading to questions about possible underreporting by schools.

Numerous reports from advocacy groups, lawmakers and government investigators have cited cases of injury and even death resulting from the use of restraint and seclusion. Currently, however, a patchwork of state rules govern the practices.

Efforts in Congress in recent years to establish nationwide standards limiting the use of restraint and seclusion in schools have stalled. While favored by disability advocates, groups representing school administrators have fought proposals to restrict the practices and insist that restraint and seclusion are only used as a last resort.