Thirteen players at training – what game structure works?

With 13, I would keep everybody active rather than force a full-size game. For a 75-minute session, you could begin with three teams of four plus one neutral in a possession game. Then move to 6v6 plus a neutral in a directional practice. If you have a goalkeeper, they can support both teams in possession or protect one large goal while the other team attacks two targets. Finish with 7v6 and change which team has the extra player. That gives you realistic overloads, midfield relationships and transition moments without one child waiting. I would not worry that it is not a full 9v9. The format is the matchday maximum, not a requirement for every practice. The key is choosing numbers that repeat the problem you want while giving every player regular decisions and involvement. Try 12 minutes of three teams of four plus a neutral, 25 minutes of 6v6 plus the goalkeeper or neutral, then 25 minutes of 7v6 with roles changing regularly. Allow the remaining time for arrival play, short explanations, drinks and a final review. Keep the same theme across all three blocks.