Living and riding in NYC I've had plenty of lock experience, including about a decade before the advent of U-locks. Your leave a chain at school is basicalkly sound if the school staff doesn't decide to cut off an "abandoned" chain. When I lived in Manhattan some friends and I got together and bought 20 chains and keyed alike padlocks, and spread them around our favorite destinations. So for the weight of a key we had 20 secure places to lock our bikes.

Mixing technologies also helps because thieves are less likely to be equipped to handle both.

But ultimately locking a bike is like the divers joke about sharks. "you don't have to outswim a shark, you just have to outswim another diver"
So the key to how much security is comparing the attractiveness of the bike, and the level of security you have to what else is out there. The goal isn't to stop a thief, but to have him decide there's easier pickins elsewhere.

BTW- I don't know about Portland, but one of the most commonly stolen items is the seat and post. That's why serious messengers and commuters in big cities use a seat leash of some kind.