Helping your students to perform to the best of their ability is always a good thing. If that requires rescoring some parts, do it. When I was teaching, I did it many times and never notated anything in the scores. I condensed trumpet and cornet parts to just 3 trumpet parts, had a clarinet play a trumpet solo, and even rescored an entire 32-bar phrase. The only time I even had a comment from a judge was when I substituted a clarinet for a short trumpet solo in the 3rd movement of Jager's Third Suite. One judge asked on the comment sheet whether it was supposed to be a trumpet solo. I didn't make a 1 that day but it wasn't because of the clarinet playing in place of the trumpet.