The Sumner County Middle School, where my 13-year-old attends, had a few cases earlier in the year. We received an information letter. It has now all blown over. As a side note and hopefully some comfort for you...I have a 16-year-old with a disabling case of Atopic Dermatitis, who is treated by a team of physicians at Vanderbilt Childrens Hosp. He has had several cases of Staph, when this occurs the doctors treat him very aggressively because of the inherent danger of his illness. However, none of the other 4 members of our family have ever had a problem. Good hygiene and resonable care keep it from spreading. Try not to worry...perhaps you can get some pocket-size kid-style hand sanitizer, they have it at Dollar General store, and send it to school with your son.
Hope this helps ease your worry