Effect of spyware and adware

In a case that has caused a lot of disquiet, a teacher has been convicted of exposing seventh-grade students in the US to pornography and could face a long sentence. Read the CNN article.
She has been convicted of having a computer in the classroom in which students were able to see ads showing inappropriate content.
Her defence: When she was not in the classroom, some students clicked on a link for some hair styles that also loaded spyware and adware pop-ups on the computer, and since the computer did not have a firewall (and probably not having the latest patches), these pop-ups started appearing faster than she could control them. A pretty strange case, after all, if by mistake you land up on a site, and you do not have pop-up protection, then the number of pop-ups that can get opened can be over-whelming.
One main point that emerges from this case is that it is absolutely necessary that a machine be running a firewall and has all the latest patches.