One think you might want to keep in mind is not to make it to analytical. By that I mean don’t go to deep into explaining every single muscle that is involved in the lifting movements because at that age the majority of them don’t have the interest nor focus to learn human anatomy. You need to keep it very general and focus more on technique and execution.
As for way to make it fun I’m not sure there is any good way to approach it. If it’s football players (or athletes in general) you can add some competition games among them or split them into team and have them compete in various challenges after each class (or every other class). Pushups, running, team building challenges, etc. are some of the things you can incorporate into your teaching style.
I hope this helps.