checking the internet history

Today I was checking the internet history ( to see what sites my kids are on) so one of them searched for porn no one actually watched anything just was on the site...( I thought I filtered out adult sites but it only blurs them from the bing homepage) According to the times it's when my oldest daughter got home so I had a talk with her, she said that a girl in her class asked her did she know what porn was ( my daughter is really shy and just stared a new school) she said she told the girl no so the girl wrote down a site and told her to go look at it... she showed me the paper definitely not her hand writing. So I asked her about the site she stared crying and said that she typed it in and seen inappropriate stuff and quickly closed he page.. I told her those sites are off limit because they aren't what sex is about she was like mom please I don't want to talk about that stuff... it was nasty and made me uncomfortable... my thing is should I go talk to the principal!? She said that the girl always talks about partying and having a boyfriend they are only in the 5th grade