First Ptdi at
www.Ptdi.org is not a school but a certification. To me, the biggest thing is time at the controls of the vehicle. Time in the truck when you are not the one driving does not teach much. Ptdi certification requires a specific amount of time for a student to be at the controls of the vehicle. Driving a tractor trailer is a learned skill that, at least in my opinion, you have to experience first hand and learn by doing. Again, watching someone else do the driving while you sit in the rear of the cab don't teach you squat.
I suggest you check out
www.Ptdi.org and find out more about Ptdi and their standards.
Get the best quality training you can get. Some day you are going to be solo in the truck and the only thing to keep you out of trouble is your skills from school and shear dumb luck. At that time, your training will be worth every cent you paid and then some.