When I was in 5th grade a kid named Chris went to some research sites on one of the computers (Unfortunatly, this is the one our teacher used for web-surfing). It took a while for everyone to realize what site he went to. After that, we freaked out because without the computers, indoor recess would be boring.
We ended up Clearing Browsing history, cookies, and all of that stuff. We thought we were in the clear, untill one day, a pop-up came up with a whole bunch of naked women on it. Most of us saw it too, since the teacher was doing something on our schools website. We all just stod in silenece for a minute, untill the teacher said something along the lines of "Who was visiting innaporpate websites?" Nobody fessed up, so we lost the computers for the next 2 months. Apparently, we got lots of ad-ware and some trojans from that.
I have more stories like this (Kids at my school were always messing around with computers)
Also, I use Malwarebytes.