Trying to solve this at the application level is wrong, as there are far too many workarounds (manually clearing or disabling the history in Firefox, other browsers with private browsing, embedded browsing components in other applications). In future versions having the Windows parental controls enabled will disable private browsing, but by far the best method of spying on your children without them suspecting you are is to log the network traffic either via your router settings or by using Wireshark.

I work at a school and am busy trying to find and block the access of students.

Having private browsing is a REALLY BIG NUISANCE because it makes tracking their behavior much harder, which makes controlling the websites and preventing future access much much more difficult, too.