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Someone on Reddit found the same movie.
https://www.reddit.com/r/tipofmytongue/comments/3511tu/tomt_movie_80s90s_drama_can_only_remember_one/
Remember any names (other than Roy)? I presume the girl was white because her uncle was the one that slammed the lid. The Reddit version said it was the husband of the lady who told the story. You said you were absent the next day, which implies you saw it in school. Which class were you in? Was the movie based on a true story?
The TV movie Hope from 1997 http://www.imdb.com/title/tt0119312/?ref_=ttpl_pl_tt
has a girl with a racist uncle Ray, which is close. The person on Reddit thought it was Ray (the movie about Ray Charles), but that came out too late. It also says there is a female dance teacher in Hope that could have remembered the piano incident.
Here's the full movie of Hope, you can check to see if it matches what you remember: https://www.youtube.com/watch?v=z_7ieQ13ycY
EDIT: I searched through the subtitles and confirmed that the scene you described appears in the movie. Solved!

This movie from The '80s or The '90s that had to do with racism and the Civil Rights struggle. All I remember is 1. the protagonist is a girl 2. the villain is her uncle or father. 3. At one point the girl's mother talks about a time when her uncle slammed a piano lid on this black kid's hands and broke them
Edited by Bootlebat