Security will remove the individuals who are not on campus for bona fide reasons. If you call security services first, and it is a police matter, security will contact the appropriate law agency on your behalf. Make notes of any incidents to assist security or the police in their investigation. If not actually attacked, but you are confronted by someone with a weapon, you should comply - don't resist, don't block an attacker's escape route. Resistance that is loud, clear, firm, and early, deters many would-be attackers, particularly those without weapons.
Some authorities say it is best to yell or roar, rather than scream.
The choice of whether to physically resist an attack is up to you - there is no way to predict an attacker's actions. If you choose to physically resist, take advantage of any weapon you may have, such as a book, backpack, umbrella, keys, etc. The most common crime is theft of personal property such as wallets, handbags, carrying bags of all kinds, and theft from automobiles. It takes only seconds for a thief to steal your valuables. To guard against theft is to discourage would-be thieves.