If you covered the backing manuevers in school, I would go ahead and take the test in the rental, all you can do is fail at the worst. it is not rocket science, and there is a very good chance you wind up with your cdl.
At the very least, you will know what to expect and how to pass the next time.
I posted this in another thread. We have a member here who got his cdl on his own then went to drive for Western Express. They provided him training, then he was in his own truck runnin freight.
Speak to WE, there's a good chance you can get in with them.