The school had to close the week before Halloween to bring in pest controllers to rid the school of false widow spiders.
The school has tried to downplay the infestation as an "inconvenience," especially as no one has died as a result of the spiders' bites. A 39-year-old Essex soccer player nearly had to have his leg amputated after a bite earlier this year, however.
The arachnids are commonly mistaken for the related black widow spider, whose bite can be fatal. Some scientists suspect the spiders have moved into new areas because of climate change.
The false widows are still venomous, however, and are the most dangerous of the 12 species of biting spider in Britain. But unlike the black widow, the false widow's bite is not deadly.
"Generally speaking symptoms are no worse than a bee or wasp sting and in truth you're more likely to be stung by a wasp than you are to be bitten by this spider," said Stuart Hine, manager of the Insect Identification Service at the Natural History Museum.
The spider is "not aggressive" to humans, and the chances of being bitten by one are extremely low. If they do bite, symptoms include severe swelling, chest pains and tingling of the fingers.
"When people do get bitten, and this is very infrequent generally speaking, it's because they've got inside clothing and they've been pressed against the skin when we put that on, or occasionally in bedding. That's the only circumstance that they bite," Hine said.
The Dean Academy said it "had identified an issue with false widow spiders" on campus and closed Wednesday. School officials expect the school to reopen Thursday.