Private schools are good in that they are for-profit institutions that won't make money if they don't deliver a good education. They're bad for the same reason: They'll cut costs and maximize profits anywhere and everywhere. If you attend one, be sure to ask how much driving time you'll get, and inspect its facilities.
Local and state governments also fund truck driving schools that are run from community colleges and trade schools. These can be cheaper than private schools, but also longer and less flexible. For example, if a 15-week course starts on the first of a month, and you don't register until the second, you're going to have to wait 15 weeks. Length can be a problem, too, as your financial status might not allow you to wait 15 weeks to get your CDL.
Motor Carrier Training is run by private companies in dire need of drivers. This is quick and brutal on-the-job training, designed not to make you a good driver but rather just to give you a bare minimum pass on the CDL test. You'll be able to work as a truck driver after completing one of these programs, but the odds are against you being a good one.
To gauge whether you should attend a school, check to see if it's certified. Certification is a voluntary process carried out by the Professional Truck Driver Institute (PTDI). The PTDI certificate is a good way to gauge a school's quality because it requires--among other things--44 hours of driving time. If you can't afford or find a PTDI school near you, use the 44 hours as a benchmark. Forty hours is OK, too; 16 is not.
Even better than certification is accreditation. This is not done by a private institute but rather by the U.S. Department of Transportation, and attending an accredited school gives you access to federal student loans, Pell grants and related federal aid.
One last thing to be aware of is the difference between driving and observation. Some schools will advertise 40 or more hours behind the wheel, which sounds great, but it often doesn't actually mean that all that time is driving time. Rather, it's not at all uncommon for schools to economize by putting four people in a truck at a time and rotating who gets to drive. When you're not driving, you're "observing," which does not actually teach you anything about how to drive. After all, you probably spent years as a passenger in cars, none of which helped you when you turned 16 and began driving yourself. So when you're investigating schools, be sure to ask about how much driving and observation time you're paying for, as only the former will actually benefit you.