“We have received an overwhelming response from staff indicating that they do not feel safe returning to classrooms with students,” Gregory Wyman, superintendent of the J.O. Combs Unified School District, said in a letter to parents on Friday. “In response, we have received a high volume of staff absences for Monday citing health and safety concerns,” Wyman said.

The district’s governing board had voted to resume in-person education on Monday despite failing to meet benchmarks recommended by the state, the New York Times reported. But the district announced Friday that all of Monday’s classes, including virtual learning, would be cancelled after 109 staff members called in sick, the Arizona Republic reported.

“Due to these insufficient staffing levels, schools will not be able to reopen on Monday as planned,” Wyman said in the letter.

Wyman also told parents that he was unsure when classes would resume because the district could not predict how long the absences would last.