I remember this as a short film I saw in elementary school.
On some planet somewhere, or some future world, the sky is permanently overcast. There are all these kids, like in a school, but with (as I remember it) zero supervision. A girl who believes that the sun exists and will shine one day, is mocked and tormented by her classmates who know better. There's no such thing as the sun, they tell her.
They lock her in a... room of some type. And they're running around outside playing while she's curled up and crying in the room.
Suddenly the clouds part and the sun! there it is! shining down! And all the kids are amazed and happy and laughing!
And the girl in the... room... (and this is the only part I remember visually) sees a beam of sunlight coming through the crack of the door and holds her hand out into the beam of sunlight and smiles.
What is this I'm remembering???