"I was backup QB during the 'season' between sophomore and junior year, so I will tell people I was backup QB during my sophomore and junior year."

Also, I HAVE played football, and I've never heard anyone claim that summer camp was a season in and of itself, and that they started/ were backup for that season. When someone says "I started my senior year" they mean during the football seson when we actually played other opponents in full pads, you know, when it really counts, I was the starter."