I can't speak for the States, but here Campus "Police" are special constables...i e, they passed the physical component of the Police test and that's it. Call your local Police!
If you really think you are an easy target, there are some things you can change...1) Don't talk on your cell phone when walking to your car, it makes you distracted and you don't pay attention to your surroundings.. .2) Put your car key/ house key between the knuckles of your first two fingers, chances are you'll draw blood if attacked....3) Canadian campus' have safe-walkers to walk you to and from classses, cars... late at night, if you have them, use them!! Pepper spray is illegal in Canada (don't know about the states) and truely if you have never been exposed to it you may become more of a victim---it BURNS,I can't even explain to you how much, you can't physically open your eyes, takes at least 10 mins to keep your eyes open and navigate the world by yourself again, it drops people to the ground (good for attacker, not for you), and it is a spray, which makes it difficult to use outdoors, chances are you will get some on you too.
And don't get down on yourself, your screaming did the trick!! So what if you didn't beat him up, what would you do after you did anyway?