My thoughts as to why punishment should not be allowed in the classroom or in school are becuase I believe that it is the parents job to discipline the child. A school teacher has so many students that they can't possibly care abou teach and every one the way that a parent does. Therefore I believe that they could abuse that power given to them and not inflict punishment in always honest ways. The love that a parent has for a child is caring enough about their child to implement a consequence. To care enough about their child to be honest and do what is best for them. That relationship cannot always be found between teachers and students.
This is just my opinion, hope it helps! If you have any more questions or would like further explanation just let me know! :)