Please provide the statute that says a cop, or anyone else, can grab you as you are leaving a building you were told to leave. Cops have no more right to grab you than anyone else. The student was doing what he was told to do. Grabbing him at that point was battery. Bullsnot.
Show me where a police officer does not have the right to grab your arm and escort you from a premesis upon which you are trespassing?
California Penal Code section 242: "A battery is any willful and unlawful use of force or violence upon the person of another."
Now tell me, are you a first year law student or just a naive anarchist?
There is no claim by anyone except you that he was trespassing. Even the cops don't claim he was trespassing. At worst he violated a school rule.
By the account we have he was leaving as instructed by the CSO. The UCPD cop had no lawful right to grab him and stop him from leaving.