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.
Lisa is the only one in the gang never seen competing in the school song contest, despite being shown to be a skilled musician (singing, playing piano and guitar) and competent songwriter before the contest is even announced. At no point is it ever explained how Zack sabotaged her, either, as he apparently sabotaged everyone in the gang except for Screech.