Officer Rick Moore of the Oakland school district police arrives at a high school following reports of a fight Dec. 17, 2012, in Oakland, Calif.Noah Berger/AP
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First designed to protect recently desegregated schools, school police officer presence has grown in recent decades. Across the country, data shows that students of color and those with disabilities are disproportionately referred to law enforcement in schools and advocates are calling for reform.

Click through this timeline to learn more about the origins and expansion of law enforcement in the education system.