I thought there was a such thing as 5ft. 12in. until 5th grade
Why were you thinking about that in 5th grade?
That's when I learned about it and had the realization.
The school should have taught you much sooner than that about measurements. It is good to realize when you have made a mistake and apply the new knowledge.
I had a whole bunch of other stuff on top of my mind. I won't make that problem again.
That is okay that it happened. It's wrong for schools not to teach children the right way. I wouldn't say you are to blame only the school for not thinking about this sooner.