I will ALWAYS recommend Jansport backbacks - they are hella durable and will stand the test of time. I've never thrown any of mine away and I still use my one from kindergarten. That thing went through 8 years of summer camp and is still decent.
I really like the one I have now though. I bought it for college. 6 years later it is still going strong. I don't know what the make is but it was like a mid-level quality back pack from their line and I can definitely vouch for its durability. I've jammed it full with textbooks, laptops, art supplies, all my snowboard gear, taken it hiking, smuggled alcohol onto my college's dry campus with it.... I use it for work now and it still works like a champ. It has a buckle on the front that goes across your chest and it's really helpful when you're carrying heavy things or carrying it for long periods of time - it takes the pressure off of your back a bit. Plus, when it's buckled, it can't be ripped off your body if someone were to do the typical run-past-you-and-grab-your-purse-off-your-shoulder type of mugging. I think you could find a decent one for under $80.
As for security, you could always buy tiny back pack locks to put through the zipper pulls if you're really worried about it. With my backpack, I usually put my important things in the little pouch at the top behind my neck or in the padded section that fits the laptop that rests upon your back. You'd know if someone was digging through your bag if they were sticking their hands in those places.
ETA - also, always go with the canvas-y ones over the plastic-y ones. Canvas can take the wear and tear. The plastic will deteriorate
