A world champion is in our midst.You won't guess his sport, but I'll throw some clues out there if you want to try:1. He spends six days a week practicing at the Ridge Racket Club in Grass Valley.2. His sponsor is Wilson.3. He started playing when he was 4. Check that - he was 12. Or was it 15?4. The sport he plays has no United States national team.Stumped?Johannes Schubert - a 16-year-old Austrian exchange student at Nevada Union - is racketlon's under-16 world champion. Racketlon is a four-event sport consisting of tennis, table tennis, badminton and squash.Schubert, who goes …