MONTPELIER, Vt. (AP) – A former Bradford elementary schoolteacher charged with sexually exploiting some of his students pleaded guilty Friday to two federal child pornography charges that could send him to prison for at least 25 years.

Richard Foster, 52, entered the pleas in U.S. District Court in Burlington. Sentencing is set for Dec. 22.

Prosecutors say they will ask for a sentence of 25 to 27 years in prison. As part of a plea agreement, Foster agreed not to ask for a sentence of less than 25 years.

Foster was arrested a year ago after the mother of a 12-year-old Florida boy who had lived in Bradford found sexually explicit photo text messages Foster sent to the boy. Before he was taken into custody Foster tried to commit suicide.

Investigators later found “massive” amounts of digital evidence as well as a CD which depicts Foster and the boy engaged in sexual activity at Foster’s home. They later found additional images of Foster with another boy, prosecutors said.

Foster taught in the Orange County schools for two decades and was a sixth grade teacher at the time of his arrest a year ago. He was originally charged with state crimes, but the case was taken over by federal prosecutors.

Before his arrest Foster had no criminal history. Bradford school officials said they had no evidence of his being involved with boys. But after his arrest many parents said there had been hints Foster was inappropriately involved with children.

School officials could not be reached for comment on Friday afternoon.