Offside trap at U13 – useful or unnecessary?

I would teach the back line to recognise offside, but I would not make “catch them offside” the main behaviour. The useful defensive picture is stepping when there is pressure on the ball and staying connected as a unit. Sometimes that will naturally leave a forward offside. If the defenders are stepping purely because they hope the assistant raises the flag, they can stop reading the ball and the space behind them. In training, use an offside line but reward the defending team for winning possession or forcing play backwards, not for the flag itself. At U13, offside is part of the game and defenders should understand how line height affects it. I’d just keep the priority on good pressure, cover and distances. The offside outcome should come from those behaviours rather than replace them.