Officials temporarily shut the infested Dean Academy secondary school in Gloucestershire, western England, so that pest control experts could exterminate the false widow spiders.
Britain has this year seen a huge increase in numbers of the false widow (Steatoda nobilis), whose bite can cause severe swelling and chest pains.
No deaths have been recorded in Britain so far from the spiders, which have a distinctive cream marking on their large abdomen.
The school said the spiders were first discovered in its technology block on Tuesday, but after it was fumigated they surfaced elsewhere so they had to close the entire premises.
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False widow spiders are thought to have been introduced to Britain from the Canary Isles in the 1870s.
Colonies have been established in southern England for over a century, with repeated reintroductions in imports of bananas, according to Britain's National History Museum.
But John Tweddle, an expert at the museum, said populations appeared to be increasing and spreading, most likely as a response to global warming.