It wasn't down to any teachers, it was a scientific truth based on the information of the time. Now that information has been changed and the truth has changed right along with it (ie, we now have eight planets in our Solar System). See post #45.
BTW, your post makes my point. If we find more planets in our Solar System, then our truth about the system will have to change.
I think I agree with you here
If you had asked anyone back in 1995 “Is it true that Pluto is a planet?” they would have said “Yes”. It was a truth.
I wouldn’t phrase it necessarily as “scientific truth” though, since that’s not really a thing.