All accredited means is that the school meets the minimum requirements of some organization.
Not going to a PTDI school is only going to close out a couple of job opportunities. The one real advantage to going to a PTDI school is that you have a better idea what to expect. In order to get a PTDI certification a school has to have a minimum amount of classroom, range, and actual behind the wheel on the road hours for each student. There are also requirements about what the school must teach and cover during the course.
The total min hours for a PTDI school is 160 hours. As I said before most companies do not require a PTDI course, but many want to see at least a 160 hour course.
There are many schools out there that meet or exceed all requirements of a PTDI course but simply do not want to pay the fees for the certification process. The most important thing is quality of instruction.
If you go to
www.ptdi.org or
www.truckschoolsusa.com you will find some good info in what you want to look for in a school program.