Tense consistency

[lariral]

Hello,

I have a very faint memory of the fact that once you start a sentence with a specific tense, you should keep that tense as long it doesn't interfere with the meaning. A specific example would be:
"I CAME across Digital Marketing and realised that everybody needS online promotion but it’S a subject that schools all over the world ARE not yet teaching."

My gut says that it would be correct:
"I CAME across Digital Marketing and realised that everybody needED online promotion but it WAS a subject that schools all over the world WERE not yet teaching." So although everybody needs it now and it is a subject not taught in schools, these should be in the past because of the "came" in the beginning.

If it's correct, I can't explain why. Could you please help me out?

Thank you very much!

[Firefox]

I asked EnglishClub's grammar guru a similar question here:
viewtopic.php?f=199&t=77770