(CNN) Fifty-four teaching assistants at the University of California, Santa Cruz were fired after they refused to turn in final fall grades as part of an ongoing strike for higher wages.

"It is extremely disappointing to us that we have to take such a drastic step, but we ultimately cannot retain graduate students as teaching assistants who will not fulfill their responsibilities," Lori G. Kletzer, the school's interim campus provost and executive vice chancellor, wrote in a letter to students and staff on Friday.

About 200 graduate students at the northern California campus starting withholding grades in December, demanding an increase in wages of $1,412 that would help them cover rent near the school. Earlier this month, the students started holding protests on campus and many stopped teaching, holding office hours and conducting research.

The strike, which is not authorized by the union that represents the graduate student employees, is in violation of the current bargaining agreement, the university said.

UCSC President Janet Napolitano and other university officials have continuously expressed their disapproval of the student's actions. In a February 14 letter to the university community, Napolitano said she won't accede to the student's demands because it would "undercut the very foundation of an agreement negotiated in good faith" by the union.

Read More