More info from the US Dept of Labor website on bus drivers:
Many employers prefer applicants who are at least 24 years old. Because bus drivers deal with passengers, they must be courteous. They need an even temperament and emotional stability because driving in heavy, fast-moving, or stop-and-go traffic and dealing with passengers can be stressful. Drivers must have strong customer service skills, including communication skills and the ability to coordinate and manage large groups of people. In some States, school bus drivers must pass a background investigation to uncover any criminal record or history of mental problems.
Bus drivers held about 647,500 jobs in 2008. Around 70 percent of all bus drivers were school bus drivers working primarily for school systems or for companies providing school bus services under contract. Most of the remainder worked for private and local government transit systems; some also worked for intercity and charter bus lines.