As long as he uses the time off for what it is meant to be used for, I don't see any problems with it. If he doesn't use it for the purpose of doing this school work, there should be some unpleasant consequences!

Are the others in the group going to take time off from school also, so they can practice their presentation as a group? If not, what good will it do to have your son out if the others aren't able to meet with him to practice?