This film was (I think) a TV movie from around 25 years ago. The “hero” of the film is a high school teen from the wrong side of the tracks, who falls for a girl from the other side. She sees him playing a piano in the school auditorium after hours and realises (trope) he’s a nice guy etc etc.
For some reason theres a big misunderstanding and he ends up taking the entire class hostage. The sheriff’s(?) son ends up in there as well as a hostage and the police try and get a message to him in a can of mountain dew but the hero finds it instead
Like I say, even at the time I saw huge similarities to Rage and I’m not sure if theres bits I’m misremembering because of that, but certain on the piano scene at least. I’ve got an inkling it had a name like Tuesday to far or SOMETHING like that, but yeah, any help, it’s been bugging me for years!!
Could it be, Detention: The Siege at Johnson High (aka Target for rage and Hostage High)?
It was made for TV, came out 20 years ago, the sheriff’s daughter is among those taken hostage. I don’t know about the piano scene, but it’s worth a try.