I usually give the horses a solid 12 months from starting the changes before expecting a clean solid change every time.
A week is a bit extreme.
There are a few exercises to try. From a left lead canter, begin a left half pass and slowly turn it into a leg yield (ie, flexed to the right) but without the horse scooting out through the left side. The body should be parallel to the longside of the arena. Then ask for the change and continue the LY for another few strides.
As well from a counter canter (ie right lead) do a loop off the wall, not quite to the center of the arena, and as you begin turning right back to the wall, ask for the change to left lead and continue turning right.
Those are my go tos for a late change. You also might need to consider that the horses aren't ready for changes quite yet.
I agree that young horses and deadlines don't go together.