All I remember about the movie is that there's a group of kids that lock a girl in a room because she tells them the sun is going to shine (which it has never done) and they don't believe her. But she ends up being right, and they end of going out to play in it and forgetting about her. So she's locked in and never gets to go out and see the sun shining.

All Summer in a Day (http://www.imdb.com/title/tt0195517/reference)

Thanks, I was beginning to wonder if I'd dreamt it... found it on IMDB and saw it was a TV short! Thank you!