WASHINGTON — A teacher at a D.C. charter school has been charged with sexually abusing a teenage boy.

The Montgomery County police said in a statement that Howard Alpert, 73, of Bethesda, was arrested Friday morning and charged with a fourth-degree sex offense and sexual abuse of a teenager he had been paying to do yard work around his house for nearly two years.

Alpert was a science teacher at the Washington Latin Public Charter School, according to a letter sent to parents.

The police said Alpert at least once had inappropriate sexual contact with the boy when he came into Alpert’s house on a break. They say Alpert also took pictures of the boy working and at least once told the boy to take his shirt off, then took a picture.

The police didn’t release any information about Alpert’s bond status.

The letter said Alpert resigned from the school earlier this week, effective Friday.

The police said the boy wasn’t a student at the school, but they’re asking anyone who knows anything about any possible inappropriate contact Alpert had with other young people to call them at 240-773-5400.