S.R.
First of all....don't assume anything. Ask the questions. Are the parents going to be home? Do they approve of your son's visit? They may prefer he not visit at their house after school especially if they will not be there. I would not.
Perhaps you should call the parents to see what the plans are and the rules of their house. I would also consider having her over to your house after you have spoken to her parents. After you have seen them together at your house with your supervision, go with your gut feelings. Before any visits, set up the rules that you are comfortable with so as not to have any awkward moments later. Good luck with this.