OAKLAND -- Montera Middle School Assistant Principal Mary Charlesworth is leaving Montera after 21 years at the school.

Charlesworth became an assistant principal one year ago after 20 years as a math and science teacher and as head of the math department. Charlesworth's contract as an assistant principal was not renewed for the coming school year. School administrators are "at will" employees, not subject to union contracts and rules.

"I am very sad to leave," Charlesworth said. "This is a painful process. I assumed one day I would leave, but it would be under my own terms. The politics of Oakland have taught me differently."

"I was shocked when I heard this news," said Marie Brady, a parent of a graduate and a seventh-grader. "She really pulled those kids together. She made things happen."

Charlesworth was part of a new administrative team at Montera this last year, including Principal Tina Tranzor and Assistant Principal Jeremy Packman. Collectively, the team had one year of administrative experience. Tranzor served one year as an assistant principal under the previous principal, Russom Mesfun, who left the school in 2012.

"Twenty-plus years of experience in one school -- that's rare. It's a huge loss," said Thom Kwiatkowski, a former teacher and assistant principal at Montera, who worked with Charlesworth for 11 years. "Wherever she lands, someone is going to be extremely lucky.

"She has certainly been an asset over the years. I don't know of anything she didn't do," Kwiatkowski said. Charlesworth's extracurricular activities over the years included being the teacher liaison for the Parent Faculty Club, after school tutoring, chaperoning the dances, student council and coaching girls' basketball.

Charlesworth, a native of Walnut Creek, began at Montera in 1992 as a math and science teacher. She first became interested in teaching after a stint with the Peace Corps in Sierra Leone. She earned a master's degree in education from Columbia University in New York City. After teaching for two years in New York, Charlesworth returned to California.

"It was a very different school back then," Charlesworth recalled. For starters, it was a junior high, with seventh, eighth and ninth grades, and a host of electives, including music, art, cooking, wood and metal shops, as well as a choice of three languages -- French, Spanish and German.

Charlesworth briefly left Montera in 1998, after a 28-day teachers strike in 1997.

"The strike was my first understanding of education in Oakland," she said. "The low pay and the not being taken seriously as a professional had an impact on the entire community. I was naive to that part of education, and it was the first time I questioned my choice."

Charlesworth took a job at a private, international school in Madrid, Spain.

"Spain taught me about the misconception of kids," she said. "I was not accustomed to the sense of privilege and lack of support for kids with special needs, which made me realize that I didn't want to work in this environment."

Charlesworth returned to a different Montera in 2000. The school had transformed into a middle school and was now serving grades six, seven, and eight. She was hired over the phone by a new principal, Cheryl Rodby, and new assistant principal, Joe Salamak.

"I was extremely ecstatic about returning to Montera," she said. In Spain, "I had rekindled a passion for Oakland. I value diversity."

Charlesworth began getting a taste for what life was like outside of the classroom, when Russom Mesfun became principal in 2007.

"I knew I could have an impact in the classroom, but I felt I could do more. My dream was to be an administrator at Montera," Charlesworth said.

Charlesworth obtained her administrative credential from California State University, East Bay, two years ago. "I was happy at Montera and wanted to wait for an administrative opening at Montera," said Charlesworth.

She got her chance last year with the resignation of Mesfun and Kwiatkowski in 2012.

"I loved being an administrator," she said. "It's the hardest thing I've ever done. I learned a lot about myself. I struggled, made mistakes, but that's what it takes to accomplish your goals.

"This job is about the relationships. I believe I was successful in the classroom and as an administrator because students knew I cared, knew I would listen and knew I held them to a high standard both academically and in their behavior.

"I loved being part of the Montera family and loved having my year of being in my dream job," Charlesworth said.