When, after two weeks of riding your bike to school, you lose your key, call your mom to come pick you up and get the spare key from that drawer in your dad's dresser that holds spare change, spare business cards, spare parts, spare wristwatches and spare keys.
Listen to the lecture from your mom about using a key chain, always keeping your key in the same place, and being responsible. Say NO when she asks, "do we need to put it on a string around your neck?"
After 2 more weeks, lose the spare key.
Bonus - How to Be a Mom: Lecturing Your Son Who Lost his Last Bike Lock Key
"Well, I guess that's it. I knew we shouldn't have bought you such a nice bike. Why can't you be responsible? Didn't you have a key chain on that key? Didn't you put the key in the same place in your backpack every day? I knew I should have made you wear it on a string around your neck. I guess your bike will have to stay at school forever now. And it's supposed to rain this weekend. I think it's starting to rain now."
Extra Bonus - How to be a Dad: Cutting A U-Lock From Your Son's Bike
Check with co-workers who will tell you about the Bic Pen trick. Look it up on YouTube. Come home and find a Bic Pen and spend 15 minutes trying to pry the end off. Drive over to the school with the Bic Pen.
Come back home, have dinner, then google how to really get a u-lock off a bike. Go to Home Depot. Purchase a converter kit for your drill that makes it into an angle grinder for $8. Go back to the school and try and cut the lock off the bike with the angle grinder and your cordless drill. When the drill battery dies, go back to Home Depot. Purchase a power converter that hooks up to the car battery to power your corded drill for $80. Go back to the school in the dark and cut off the lock.