She also used Twitter to share her philosophy on life and tough times: "Every day may not be good, but there is something good in every day."
"No matter what happens in life, be good to people. Being good to people is a wonderful legacy to leave behind."
To students in her geometry and algebra classes, she once wrote: "Full school week ahead. That can only mean one thing: lots of math fun :)"
'She cared about every single student'
When Ritzer took the job at Danvers High School, it meant she could live with her parents in Andover about 15 miles away. This would save money as she also worked on a master's degree in school counseling at Salem State University.
"As a dedicated teacher, Colleen wanted to work with and help children with special needs," an email from Salem State University read. "She believed children have much to offer and often do not realize how special they are as individuals. In her application to Salem State she said she was dedicated to 'helping students in times of need.'"
After graduating magna cum laude from Assumption College in Worcester, Mass., in 2011, her first teaching job was at a middle school.
Former colleague and friend Charlotte Dzerkacz told CNN on Wednesday that she knew after just her first few weeks that she wanted to be a teacher for the rest of her life.
"She was energetic and compassionate," Dzerkacz said. "You couldn't ask for anything more from a teacher or a friend. She cared about every single student and put in many hours after school every day, always thinking about how she could be better and better help students. She was truly a beautiful person."
Ritzer "always had a smile on her face" and "was extremely approachable, so I don't know how she could be the target of something like this," Dzerkacz said.
Her students were shattered by the loss
"There's not words to describe her," freshman Spencer Wade said. "She's such an excellent teacher."
Kyle Cahill, another Danvers student, said Ritzer was the kind of teacher who would go the extra mile for her students and would routinely stay late after school to provide extra help.
When student Alex Taylor celebrated his birthday last week, Ritzer tweeted to him: "Happy Birthday! Hope you had a great day :)"
Taylor tweeted about her Wednesday: "Actually unbelievable, one of my favorite teachers ever, a great teacher and a great person #RIP"
Chris Weimert said he was just visiting with Ritzer in her classroom on Monday.
"It's just surreal that how quickly someone can go, and how much we take for granted everyday," Weimert said.
Just days before this school year began, Ritzer posted a quote from Ralph Waldo Emerson on her Facebook page. It resonated Wednesday: "To know even one life has breathed easier because you have lived. This is to have succeeded."