Here is a list of some of the popular websites that get requested for unblocking/blocking in schools
https://drive.google.com/open?id=1o7oFLqnwOVmG135PTruhf_ghWpGi50uCXNY4KIObPtc
NOTE: It is also advisable to do a "Benefit Analysis" for all website changes, to make sure that everyone is aware of the impact (for example unblocking "googlecache.google.com" can result in this being used as a proxy bypass). A template of a Benefit Analysis can be found here:
https://drive.google.com/open?id=1NQVBX_Mw0FyLaHJvmWt52IXBylDXw6iko4WwxGUa9JY