Re: Writing a story.
However, this is one case in which you don't need to stick with the past.
It's a "universal truth" type of thing, and you don't have to put it in the past, just like you don't have to backshift in reported speech when it's a universal fact.
The teacher said, "The Earth is round."
The teacher said that the Earth is round.
It is said that... is on the same level.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.