Rhaedaoui was sitting two seats away from Chism, 14, who she said had remained quiet in math class and shunned group work in class, preferring to work alone.
"He just didn't raise his hand once, didn't ask any questions," she said.
On Tuesday, while his classmates completed worksheets, Rhaedaoui said Chism instead chose to draw in his notebook.
Ms. Ritzer noticed, but instead of getting mad at the student for being off task, Rhaedaoui said she expressed interest in the fact Chism could draw.
"She connected with all of her students, so she knew if something was wrong or if you needed help with anything," Rhaedaoui said.
Investigators say Chism killed Ritzer around 3:30 p.m. Tuesday in the school's faculty bathroom and that surveillance cameras show him using what appears to be a recycling bin to move her body to the woods behind the school.
Police are still trying to determine a motive for the murder of Ritzer, 24, a well-liked teacher.
After he allegedly killed Ritzer, police said Chism then went to the Hollywood Hits Cinema.
Chism was charged as an adult on Wednesday. The court entered a not guilty plea on his behalf.
Police discovered Ritzer's blood in a second-floor bathroom late Tuesday night before her body was found behind the school, Essex County District Attorney Jonathan Blodgett said Wednesday. Chism was arrested early Wednesday morning.
Chism recently moved with his family from Tennessee to Danvers, Mass., and joined the junior varsity soccer team at the school.
His grandmother told ABC News today she feels "sadness and regret" that her grandson could have perpetrated such a grisly crime.
Angelia Chism, of Tampa, Fla., said she could not remember the last time she saw her grandson.
"We very much want to express our sadness and regret for this," Chism said.
She described her family as religious and said her son, Philip's father, is separated from the teen's mother and is temporarily staying at her home in Florida.
Public schools in Danvers were closed Wednesday. All schools but the high school were set to hold classes today, while the high school was scheduled to re-open for two hours so students could meet with counselors and grief specialists. All classes were set to resume Friday.
A prayer vigil was held Wednesday night in Danvers outside the high school as hundreds of students and members of the community remembered Ritzer as a respected, dedicated teacher.
"There's no words to describe her. She's such an excellent teacher," said Spencer Wade, a freshman at Danvers High School. "She did so well to explain everything to me. She'd have a picture of the day, cute puppies or whatever, and fun facts and she was just an excellent teacher. I don't know why she had to die."
Many at the vigil wore pink, Ritzer's favorite color. They prayed, sang and, at the end of the vigil, they placed their candles along with some stuffed animals in the middle of a ring they have formed for the gathering.
"She was just a young, caring girl that had the whole world ahead of her. And to be taken so early is just horrible," said Ritzer's uncle, Peter Martellucci.
Twenty miles south of Danvers, in Fenway Park, the Boston Red Sox had a moment of silence for Ritzer before Game 1 of the World Series against the St. Louis Cardinals.