MANITOWOC, Wis. (AP) — A Wisconsin middle school was closed on Friday after a student and a teacher were bitten by spiders.

Dozens of yellow sac spiders have been found at Wilson Middle School in Manitowoc, according to a letter from Principal Cory Erlandson. Exterminators were called in and WLUK-TV reported that more than 30 spiders had been killed by Friday morning.

Superintendent James Feil said classes at the school were dismissed early Thursday after the student was bitten. Feil said the student brushed the spider off, but the bite area became itchy and swollen.

A school nurse provided medical attention. A teacher was also bitten by a spider and needed medical attention for swelling, itching, and pain.

A letter sent to parents advises that yellow sac spider bites are not lethal but can be painful and resemble an allergic reaction.

Feil told WBAY-TV that he doesn’t know how the spiders got in the school, but he is glad the issue is being addressed.

“We had to make some immediate action,” he said.

Erlandson said classes are expected to resume Monday.