More than 21,000 door alarms have been installed in 1,200 city school buildings to prevent children from wandering off, officials told The Post.

The city Department of Education put in the alarms at a cost of $5.5 million under “Avonte’s Law,” which the City Council passed after Avonte Oquendo, an autistic 14-year-old, walked out of a side door left open at his Queens school in 2013. His body was found on a beach three months later.