Messages from my phone show up on my macbook air My Macbook air keeps getting my phone messages but since the computer is a school computer it doesn't allow me to go into iMessage. 

What should I do?
it is something you put in since you had that machine -- this is keying it to your account (without even the need for the application).

open system preferences/INTERNET ACCOUNTS, look through each you have set up and delete what you can -- or what is related to your messages account

this is the central directory for accounts like messages (could also be controlled or synced through icloud).