Last Thursday my brother and I went to a class where we learned how to make foam swords. First of all, he showed us the items we were going to use to make our sword. The inside is a PVC pipe and the outside is a foam swimming noodle. The reason why it is foam is so that when you hit someone with it, it won't hurt.
If you were to make one, you would cut the noodle so that you still have a short piece left over for the bottom of the handle, the pommel. At the top of the handle you will put another part of the noodle so that you have a hand guard like it shows in the picture. Next, you would wrap it in duct tape like in the picture to keep it together; but don't put too much on, because then it would hurt when you hit someone. Trust me--I know from experience. Then, if you like, you can decorate the handle like I did with mine. And now you have a sword that you are free to hit anyone with, without getting in trouble, because it won't hurt them!