You don’t have to. In Lync we have the option of setting the enhanced privacy mode where only people in your contact list can see your presence. To do this you have to change the CSPrivacyOption by set-CsPrivacyConfiguration – EnablePrivacyMode $True. It’s important to note that these settings are not honored by earlier client versions. Also – if the user logs in with an earlier client than these settings will be reset.
I wouldn’t do this in a lot of scenarios as this would polarize the environment. Who is considered special enough to warrant this and who isn’t. If you have well defined school policies on who can block and who can’t then this provides an option for you. Restrictions also prevent a lot of the adhoc collaboration as you will not be able to communicate with these people. I would test with your policies and see if this makes sense. Great it’s there but we don’t enable at Microsoft as we have an open door policy and I can communicate with anyone in the company via UC.