How does one simply pull up a child porn site?
Probably just Google "teen porn" (or anything more explicit) and you'll likely get your cache filled with thumbnails of teen porn... some of which may very well be of illegal age.
Any student could probably sabotage an unattended teacher's computer in under a minute.
This article says they found 20 images on his computer. Usually in these stories, they find thousands and thousands of images. 20 doesn't sound to me like someone who does this sort of thing often... which makes me suspicious.