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MANITOWOC, Wis. – A Wisconsin middle school reopened Monday after the discovery of dozens of yellow sac spiders on Friday.

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The spiders were found at Wilson Middle School in Manitowoc, according to a letter from Principal Cory Erlandson. Classes were canceled after a student and a teacher were bitten by the venomous arachnid, WLUK-TV reported.

At 2 p.m. CST Thursday, a student in class brushed a spider off her arm after being bitten, WBAY-TV reported. The girl’s arm became swollen and itchy, according to the television station.

A teacher was also bitten by a spider and needed medical attention for swelling, itching and pain, WBAY reported.

The spiders were concentrated in an area near the school’s library and also in some classrooms, according to WLUK.

According to Pennsylvania State University’s Department of Entomology website, yellow sac spiders are aggressive and may bite humans more than any other spider.

Valley Pest Control was at the school at 6 a.m. CST Friday to eradicate the spiders, WLUK reported. A news release from the school district says the crew killed close to 30 spiders.

Superintendent James Feil said he did not know how the spiders got into the school.

“That’s obviously something we’d like to determine, but whether or not we can or not because we took measures to eradicate it,” Feil told WLUK. “No, we don’t (know), except that we’ve not experienced this before, at least not anything like this degree where we had a number of spiders that we recognized. We had to make some immediate action.”