In the last season the girls sang so well together that Zack was trying to get them a recording contract. But for this episode it's only stated that Jessie sings well and Kelly sings her scale off-key.
In all other episodes when Zack calls a "time-out", everyone is frozen in time except Zack. This time, however, everyone except Screech is affected.
There are many inconsistencies when Zack calls time out at the glee club performance. Mr. Tuttle was facing forward and in the middle of a sentence, having said, "For those of you who aren't familiar with the term." When he is standing there frozen, he is suddenly facing to his left. When Zack comes back on stage and calls time in, Mr. Tuttle is talking to someone off stage and he never finishes his sentence from before the time out was called.