I would change the groups more often than the tactical objective.
If the same players are always “starters” against the same “subs”, the bib colour starts carrying a message even when the coach never says it.
You can still rehearse likely units, but use short blocks. Work the expected back four together for ten minutes, then mix the game. Pair a regular starter with a player who is currently outside the team and give both the same responsibility.
I’d also avoid language like first team and second team in training.
Players notice patterns quickly.
At U16, selection is becoming more competitive, so the environment needs to make clear that training is where positions can change, not where the hierarchy is rehearsed publicly every week.
There will be moments where you need your likely XI together.
Just make sure those moments do not become the whole session.
A player on the bench Saturday should still feel central to Tuesday’s learning.
