Zack sabotages the school song competition by detuning the piano and we see Slater and Tori's performance ruined as a result. However, Screech and Zack later use the same piano and it plays perfectly fine.
Despite only being given a couple of minutes to learn the revised version of Screech's song, Zack sings it perfectly during the performance.
Tori could have simply told Mr. Belding that the piano was out of tune but didn't.