Police started investigating after Ritzer was reported missing late Tuesday night because she had not returned home from work and was not answering her cellphone. Investigators found blood in a second-floor school bathroom and soon located the victim, he said. He did not say how Ritzer died.
"She was a very, very respected, loved teacher," Blodgett said, calling the killing a "terrible tragedy."
The boy also was reported missing Tuesday after not coming home from school. He was spotted walking along a road in neighboring Topsfield at about 12:30 a.m. Wednesday.
His name was not made public because of his age. He faces arraignment Wednesday in juvenile court, where proceedings are closed to the public.
Ritzer had a Twitter account where she gave homework assignments, encouraged students and described herself as a "math teacher often too excited about the topics I'm teaching."
There is no reason to believe anyone else was involved and there is no public safety danger, Blodgett said.
All Danvers public schools were closed Wednesday.
Ritzer is the second teacher allegedly killed by a student in the U.S. this week. A Sparks, Nev., middle school teacher was allegedly shot by a 12-year-old student on Monday.