Before you invest in shipping containers I'd look into the cost of portable classrooms. A shipping container is just a cold metal box, while the portable classroom usually has heating and air-conditioner systems built on, interior lighting, and windows. A lockable doorway with the option of adding a dead-bolt or a hasp make this a simple way lock the unit. You can always add a security system of your choice.
- RCT
LOL! I forgot about those, and I've moved a few of them. Good sugestion!