Skidding is not a recommended method of station keeping. What I do see with pilots new to formation flying is unconsciously holding a bit of rudder away from lead and then banking into lead to hold positions. I think newbie’s feel more comfortable when the airplane is yawed away from lead,
This is poor practice and potentially dangerous so it should be corrected ASAP. The easiest way is to make the student take their feet off the rudder pedals after the airplane is stable on the wing.