NASHUA, N.H. (AP) – A former math teacher at a Massachusetts high school was convicted Friday of 11 counts of aggravated sexual assault.

A jury found Severine Wamala, 46, formerly of Nashua, guilty of 11 counts of aggravated felonious sexual assault, each punishable by 10 to 20 years in prison, and acquitted him on a 12th charge.

His main accuser was a 16-year-old girl. Several other charges involving two other women were dropped after the women recanted statements they made to police.

The teenager stood by her original statement, and prosecutors were allowed to introduce a “time capsule” she had made which showed the abuse had been going on for years.

“This girl showed remarkable poise and bravery in testifying against Wamala, who had been abusing her for years,” said Patricia LaFrance, assistant county attorney.

Wamala took the witness stand in his own defense during the rape trial. He testified it made him sick to his stomach to think that anyone would believe he sexually assaulted the women.

Wamala, a native of Uganda who became a U.S. citizen after immigrating in 1988 to get his doctorate at the University of Massachusetts in Lowell, was suspended from his job as head of the math department at Lowell High School after his arrest. Wamala also advised the school’s chess club and organized chess tournaments in New England.

A date for the sentencing hearing has not been scheduled yet.