Denver -- Dozens of teachers in two Denver schools called in sick Tuesday in an apparent protest over contract negotiations.

School officials say every teacher at Academia Ana Marie Sandoval called in sick, including the music teacher and a librarian. Twelve of the 30 teachers at McGlone Elementary were also absent.

Sandoval Elementary Principal Debra Lucero Kraft says she found substitute teachers for almost all of the 16 teachers who were absent but had to get an administrative assistant, the school psychologist and a literacy coach to pitch in for the rest.

McGlone Elementary Principal Ann Alley-Walker says she was able to find substitutes to cover all classes.

Disagreements over pay between teachers and Denver Public Schools stalled contract negotiations on May 16. Denver Classroom Teachers Association President Kim Ursetta said the union is not encouraging teachers to call in sick.