Detroit has closed more than two-thirds of its public schools in the last 15 years, shortchanging students, neighborhoods and the district itself, finds a report released as teachers protest conditions at the schools that remain open.

The Detroit Federation of Teachers union filed a lawsuit Thursday against Detroit Public Schools over dangerous and deplorable classroom environments that it says deprive students of a "minimally adequate education."

"A School District in Crisis ," a report published last week by property mapping company Loveland Technologies, gives a fuller picture of the district's decline and the consequences for students.

The company surveyed the city's schools and researched others that have been demolished since the district was formed in the 1800s. Just 93 school buildings are currently in operation, Loveland found. Of the 195 buildings closed since 2000, some have been demolished or repurposed, but more than a third stand vacant.