Most useless thing they taught you at school?
This of course may vary depending on your age and where you live.
For me it was having to write neatly in cursive. I went to Primary or Grade school in the late 70's and finished in 1985. Computers were just starting to make an appearance in our school as I went into High school.
The teachers put such a huge emphasis on neat cursive handwriting. I, of course did not write all that neatly, according to them. When I think about all the energy and time that was expended on that now in a time when most everything is typed anyway or when we do hand write at work we print it because it is easier and faster to read it just makes me laugh.
Even when I started High School I went back to printing because it was just neater and easier.