Not the best analog because I played football a million years ago, but I saw big gains in my 40 time and vertical leap, and explosiveness in general after starting power/Olympic-style lifting as part of my team's strength program -squats and power-cleans in particular. A lot of my teammates also saw big gains.
Learning how to sprint with proper form was huge as well. A lot of kids lose time when they are too tense in their shoulders and face, or if their head moves too much. Not utilizing the arms properly -not enough or erratically- are big time drags as well. I bet there are a bunch of youtube videos out there that teach proper sprinting form. Good luck!