What is the standard rating of pvc for a sewer line to septic tank?

Since a septic tank is a private sewer system I don't know it the code even addresses it, but I would not consider anything other than Schedule 40 PVC. Septic tanks are usually fairly close to the top of the ground. Meaning that the piping is close to the top of the ground. The distance from the house to the tank is not usually that great and the cost difference between thin wall and Schedule 40 is not that great. Wouldn't want to forget and drive my truck over thin wall.

Harold's correct. Schedule #40 PVC is what you want to use. Make you slope 1/8th" to the foot. Good luck, Tom