After reading your post on IEP Legal issues I did some research and found if you sign the IEP document it is legal for the school to keep your fourth grade son for attending these activities. I am not sure if the school tried some of other ways of dealing with his behavior or not, but they are within there rights to keep him from attending these functions. Good luck to you and your son.
I agree with Pasty on the legalities of the consequences. I would also like you to consider some other perspectives. Most teachers of children with behavior challenges consider it a great accomplishment when their students are able to attend extra-curricular activities with their peers. Much thought usually goes into making the decision to use this consequence. First, will the child and other students remain safe in the environment? Second, will the environment remain undisrupted for all students? Third, did your child earn the privilege? All children are truly required to earn school privileges. Finally, is your child earning of the privilege in relation to his ability? Hope these thoughts help.