How is it cheating? Easy!

C&CR, 1111.e.4:

(4) Marking Music. Neither the director nor the students may mark on the music unless instructed to do so by a judge.

Putting an overlay on it so you can mark the music is clearly against the spirit of that rule.

If people would spend half the time teaching their kids how to read music that they did trying to find rules loopholes, we'd have a lot better musicians in this state.