You must get your CDL license before you can become a driver for charter buses. You can pick up a learners manual for your CDL license at your local DMV.
A typical diesel engine only burns a small amount of fuel when idling (think 1 gallon an hour). BY leaving the bus idling the driver is avoiding any issues associated with having the bus not restart...
You will probably need to obtain a CDL license with endorsements to allow you to transport school children. Also, expect that a background check will be done, so you need a clean record for driving...