A month after lifting its indoor mask mandate, the Mountain View Whisman School District reinstated it Wednesday, citing rising numbers of students testing positive for COVID-19 and the first known instance of the virus spreading among students in a district classroom.

On Tuesday, April 26, Superintendent Ayinde Rudolph notified families that all students and staff would need to resume indoor masking the following day. The policy will continue through at least May 13, Rudolph said. Wearing a mask outdoors remains optional.

The decision comes after the district identified a case of on-campus spread where five students tested positive for COVID-19 and another eight stayed home after coming down with symptoms, Rudolph said in an email to the Voice. This is the first time the district has had a confirmed instance of COVID-19 spreading at school, he said.

"Armed with this information, now is the time for us to adjust our safety measures in order to prevent further transmission," Rudolph wrote in a letter to parents.

In the past seven days, 26 students and five staff members throughout the district have tested positive, according to district data. According to Rudolph, case numbers have increased since students and staff returned from spring break earlier this month.

Mountain View Whisman, which has roughly 4,500 students, first removed its indoor mask requirement in late March, making it one of the last districts in the area to do so. Most other local schools made wearing a mask optional several weeks earlier.

At the time, Rudolph said that mandatory masks would be reinstated if more than 9% of its pool tests came back positive. The district tests groups of students, typically by classroom, together. If a pool comes back positive, each student gets tested individually.

This week, less than 5% of pool tests came back positive, according to data posted on the school district's website. Although that's below the 9% cutoff, Rudolph said the district made the decision to require masking to prevent possible closures, especially during state standardized testing, which he said is currently underway.

"Considering that this is the first case of confirmed spread, since we have reopened our doors, we felt that it was prudent to have all students mask," Rudolph said in an email. "With a confirmed case, and the several positive pools that were pending, it was the safest course of action to prevent the possibility of classroom, site or even district closure because of continued spread."