Nearly 30,000 professors, librarians, coaches, and other workers at California State University, the largest public university system in the U.S., walked off the job Monday in a weeklong strike to demand higher wages.

The stoppage across Cal State’s 23 campuses comes two weeks after CSU officials ended contract negotiations with a unilateral offer starting with a 5% pay raise this year, effective Jan. 31, far below the 12% hike that the union is seeking.

With the new semester beginning Monday, classes for many of the system’s 450,000 students could be canceled, unless faculty individually decide to cross picket lines.

Victoria Wilson, a part-time political science lecturer who picketed in the rain at Cal State Northridge in Los Angeles, said she’s striking for higher pay. She said her salary fluctuates from semester to semester, which impedes long-term financial goals.