This is a great find I happily stumbled on: from the UK, 1952 black and white movie Supersonic Saucer.
The story begins:
Somewhere in Great Britain, a group of schoolchildren visit the local planetarium. While looking through the giant telescope, one young lad, Rodney, sees what appears to be a flying saucer circling the planet Venus. Nobody believes him, however, and the class returns to school.
For more on the movie, including two pages full of images from the movie, visit the Kiddie Matinee site.