FBI agent installs spyware on son's laptop, returns laptop to school, and inadvertently catches school principal looking at child porn.

Unbeknownst to his son, the program captured his website visits, his keystrokes, and every email, chat, and instant message he sent and received. This was all delivered up to his dad in emails, while giving the monitored person no hint that it was doing so.

They don't have to go though some spy game.

They already networked your internet provider and can see where you have been, emails you sent, online banking, web searches and even use your web cam without activating any red flags !

Don't use the net, if don't want people to see what your doing !
They only come and take your computer later on to add to the evidence in court with the service provider records to bake you like a turkey.

Yeah it sounds more like the dad had his doubts about the principal beforehand and requested that the software remain intact for the laptop. Although he couldn't know for sure if the guy would try hiding it at home, unless the principal mentioned to the dad that his computer was messed up from viruses (which I wouldn't doubt, and it would be a pretty big hint as to what he was using it for).