I agree with most people here. The only people this will be effective against is those who don't know how to get around the blocks, and those who don't want to put too much effort into getting around the blocks. Even if you give people school laptops with programs installed to limit what can be viewed, people will always find ways around them if they really want to. Unless you can completely lock down the computer, so the student can't get access to the hardware, can't modify the operating system or any installed programs, or modify critical settings, it's not going to work. Even then, someone will probably eventually find a vulnerability in the security of the system that will let them change whatever they want.
TL;DR: Site blocking is futile.