What is disabling internet sharing on my macbook? I bought my computer from my school, where they had multiple locks and disabled functions. When I bought it they were suppose to delete all these locks, but I guess they didn't – now I am unable to activate my internet sharing like described in this post.

I can mess with the options, but I cannot click on for internet sharing.

Can someone walk me through the process of deleting this from my system so I can turn on my internet sharing?
One possibility why you are unable to enable internet sharing is your school may have used  a Profile Manager (http://help.apple.com/profilemanager/mac/2.1/#apd0E2214C6-50F0-48C9-A482-74CEA1D77A9F) to apply settings and preferences to your Mac. The benefit is settings (such as WiFi network access, software licenses, etc) present in your Mac allows you to use your Mac in school without having to remember to the settings. Disadvantage is you may have to ask your school for help in enabling internet sharing. Whether the school agrees will depend on its policy for accessing school resources even though the Mac is yours (as you stated, bought by you).