Oomis,
A rebbie is committing an assault if he hits a child (UNLESS he is actually defending himself FROM an assault by that student, chas v’sholom). If he hits that child, he is legally liable, and can be arrested.
Do you define things as secular law does, or as the Torah does? If CH”V bris milah would be banned, would you say, “A mohel is committing assault if he circumcises a child. He is legally liable, and can be arrested, and the parents as well, for allowing such a barbaric act to be done to their child.”?
The reason it is no longer acceptable to hit a child is because it doesn’t usually achieve the desired result, as a consequence, as mdd says, of yeridas hadoros. It’s not because it’s inherently wrong.