"All I ever heard is that she loved her job," Duffy said.
All public schools in Danvers, about 20 miles north of Boston, were closed Wednesday.
The high school's students were planning a candlelight vigil Wednesday evening.
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.
Associated Press writer Lynne Tuohy in Andover and news researcher Rhonda Shafner in New York City contributed to this report.