We sell the kids an upper-grade Kyrptonite lock at discounted prices (20.00). They have proven very durable, usually the theives will simply not try them. There are so many bikes locked with easily-defeated cables as to make it not worth while.
They can be defeated; we've had maybe 3 in the last 10 years. One guy used the "big pipes" method.... Another used a car jack.
They can be cut with a REALLY big bolt cutter... Not the kind you'd want to carry around.
Angle grinders work but of course take a long time and are very noisy...

Although there was that huge internet scare over the bic-pen thing.... We did not have one single theft during that period, and all the locks we had out at the time were of the "shunt-key" variety that you could supposedly pick. We are talking about many hundreds of these locks.
Again, not one stolen bike.
I still think the whole thing was an internet hoax. I and one of the other officers tried for some hours to open several different types of Kryptonite locks with such pens... No luck at all.
Kryptonite paid out a lot of money replacing these locks, which says something for the company.