Teaching assistants at the University of Washington went on strike May 15 after the school’s administration wouldn’t budge on the union’s demands for higher pay.

United Auto Workers Local 4121 in Seattle represents the 4,500 teaching assistants, tutors, and researchers at the school. The unit includes graduate workers as well as some undergraduate staff, Sam Sumpter, a union executive board member, said. The contract covering the workers expired April 30.

The strike comes days after postdoctoral researchers at the same...