Some unwanted visitors have made their way onto the campus of Ross Elementary School in Kearny Mesa.According to several parents and students, an alarming number of black widow spiders have been seen inside classrooms. One concerned parent and a student sent 10News images of a small, but potentially dangerous, black widow found on a wall at the school and another spotted crawling in a recycling bin."I haven't heard anything about that," said parent Irene Lancaster."That's dangerous, scary," said parent Jay Henderson.Henderson worries for her niece, who goes to school at Ross Elementary."She has allergies to the spiders," said Henderson.Some parents told 10News that the principal inspected eight classes recently and found at least four spiders in each of them. Knowing what they know about black widows, several parents called the school and school district to express concern."They can be dangerous and poisonous," said parent Marlene Washington.School district officials told 10News the school is dealing with an issue with black widow spiders. Last Friday, they fumigated a classroom. Everything seemed fine on Monday, but since then, more spiders have been sighted."I think the whole school needs to be fumigated," said parent Samantha McMann, whose son encountered a black widow in his classroom."It walked to the bookshelf. So, I just took my shoe off and slammed it on the spider," said student Travis McMann.County Vector ecologist Chris Conlan said bites by black widows are rare, even though the spiders are very common in San Diego. He said it's one of the few that can deliver a bite that can put you in the hospital."It would have to be grabbed and handled or otherwise felt like it was trapped so that it felt like it had no way out. And then it would bite out of panic," said Conlan.One can be bitten if a black widow finds its way in a glove or shoe and then one puts their hand or foot inside."If one gets in a desk at night and the kid doesn't know, gets in there and reaches for something, you never know what's going to happen," said one parent.With more black widows popping up, the school principal plans to meet with the district's pest control experts to determine what step to take next.Experts said if a person gets bitten by a black widow he or she should seek medical attention immediately.Bites can lead to neurological problems and breathing difficulties.