The U.S. Supreme Court has ruled that corporal punishment to discipline students is constitutional, although law prohibits its use in mental institutions, jails, State and Federal prisons, juvenile halls, and the military. Thirty States allow its use, but 20 States have abolished it. Paddling with a wooden instrument is the most common type of corporal punishment in schools, but it also includes pinching, ear-twisting, hair-pulling, knuckle-rapping, punching, shoving, and forcing students to exercise to exhaustion or to assume physically painful positions. Corporal punishment does not include reasonable efforts at self-defense by teachers against student attacks. Advocates of school spankings argue that they are effective with students who ignore every other disciplinary measure. Advocates also argue that many parents give teachers permission to use corporal punishment to discipline their children. Opponents of corporal punishment argue that it is not only ineffective in modifying the disciplined behavior, but it may compound the use of physical abuse by the parents, provide an example of using violence to control the behavior of others, and inflict psychological damage on the child. Another factor in the use of corporal punishment is the possibility of legal action by the parents of the disciplined child. Whatever a school's position on corporal punishment, a disciplinary code should be specified; the code must comply with relevant law, be reasonable and clear, receive approval from the school district's governing body, and be disseminated to all students and their parents.