How high should the back line really be in 9v9?

The line height should come from the pressure on the ball. If the opponent has time to look up and play over you, asking the back line to stay high regardless is risky. If your first line is applying real pressure, the defenders can step higher and keep the team connected. That is the relationship I would coach at U13. Use a game with an offside line and ask the defenders one question: “Can the player on the ball play forward comfortably?” If yes, be ready to protect space behind. If no, can we squeeze the pitch? I would avoid turning it into a choreographed offside line. Good line height is about recognising pressure, distance and the space you are protecting. The midfield stretching is often a symptom of the back line and press reading different pictures.