CHICAGO-- A 7-year-old Chicago Public Schools student told his mother a security guard locked him in a closet at Paul Revere Elementary School, prompting an investigation, reports CBS Chicago.

"I couldn't open the door because it was locked," William Simpson, the student, told CBS Chicago.

He said he was locked in an electrical closet as punishment for acting up at school.

William and his mother, Natasha Evans, said he was in the closet for 20 minutes, in a room the security officer calls B 12.

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Evans found that out after she received a phone call from someone who formerly worked at the school.

"They called me and told me they heard him screaming and crying to get out of the closet," she said.

Evans said she also found out it had happened before. She called police and confronted Revere Principal Veronica Thompson.

Thompson gave Evans a letter, acknowledging the parent's complaint and stating police had been called and the security officer was set to have a disciplinary conference on Monday.

Natasha Evans. CBS Chicago

So far, Evans said nothing has happened.

"I think he should be fired. That's over the top," she said.

Evans said her son has some behavior issues that have never been addressed by the school. She believes at least three other kids have been placed in that closet, including one child who has seizures.

CPS officials could not confirm that, but said they're investigating and will institute appropriate disciplinary actions, if necessary.