That is irritating, but it could probably also be a good learning experience for you as a composer. Try to get into their head and figure out why they are changing the specific places they are changing, and not other places. Are those sections a bit too difficult for them and they are simplifying them? Too repetitive, and they are trying to add more interest? Is the voice leading awkward there, and they can't hear it the "correct" way?
A lot of times younger musicians don't understand where the line is between changes a musician is encouraged to make to breathe life into a piece and make it their own, and changes that a musician is NOT allowed to make. (Yes, you can alter dynamics or tempo a bit, add a fermata, add an accent to make an entrance of a counter-melody stand out to the listener. Yes, you should add ornamentation on the repeat of a da capo aria. Yes, you can sometimes revoice a chord or two if you don't have the optimum number of players the composer originally intended for a piece. No, you can't rewrite the melody. No, you can't change the harmony just because you feel like it.) It's a frequent annoyance for directors and teachers of young players, but try not to take it too personally.