What is disabling internet sharing on my macbook?
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).