Reddit mentions of ONGUARD Bulldog Mini U-Lock (Black, 3.55 x 5.52-Inch)

So sorry for your loss. Cable locks aren't worth their weight in scrap steel, unfortunately. Mini U-locks like this offer much more protection.

Pm me if you need help finding a cheap reliable bike - lots of good deals on Craigslist.

Far too small. That'd probably fit someone around 5'. I'd say you should be looking for a 19-20" frame. Bear in mind that suspension isn't necessarily a good thing, there's a lot of poor quality suspension out there that's likely already broken on bikes in this price range, and it often isn't desirable anyway for many types of riding. Even on more technical trails, riding with straight forks can help you learn better technique with line selection and soft landings, if you want to get into "proper" mountain biking.

http://portland.craigslist.org/mlt/bik/5629556283.html < seems decent for the price, RockShox forks are generally good quality

http://portland.craigslist.org/mlt/bik/5586347419.html

http://portland.craigslist.org/mlt/bik/5610067720.html

http://portland.craigslist.org/mlt/bik/5630795888.html

http://portland.craigslist.org/mlt/bik/5612201881.html

http://portland.craigslist.org/mlt/bik/5630057428.html

http://portland.craigslist.org/mlt/bik/5615221078.html

http://portland.craigslist.org/mlt/bik/5631362683.html

http://portland.craigslist.org/mlt/bik/5592340701.html

If you're leaving it outside at school, it'd also be worth budgeting for a half decent U lock and learning how to use it:

http://www.802bikeguy.com/2011/07/the-modified-sheldon-brown-bike-locking-strategy/

Just had a quick look, this one for $17.93 from one of the sellers seems like a good deal:

https://www.amazon.com/gp/product/B005YPKL5U

Which isn't a good thing. People can burst those locks by putting a jack inside them. You want the smallest lock you can usefully use - something about this size works quite nicely.

Yes, decent-locks are sooo expensive. I live in a city with remarkable levels of bike theft. Nary a soul has touched my bulldog, in 10 years. And what the heck kind of logic is that? If you own a $1000 bike, what's more to buy a lock to protect it? Christ almighty.