Why do lockers have slots?
Why do lockers have slots? Of course this isn't secure, but gyms don't want people leaving stuff in the lockers, and so want a way to get into the locker without hiring a locksmith or having to break the lock open. As far as I know it is a 'ventilation-hole'. Behind this hole is a spring which will push the lock 'open' when you turn the key.
Are lockers better than backpacks?
In conclusion, backpacks are easier to use than lockers are. If you have a backpack you won't be late for your next class. You won't get detention because you are not late. You also will have everything with you the whole day.