For all you youngsters:
It has nothing to do with writing control, muscle development, dexterity, etc.
It goes back to the days before Ballpoint pens.
Little children could not be trusted to fill their fountain pens from the inkwells at the front of school desks. Old time desks had a cutout to hold the ink bottle. Children would make a mess with the ink. Girls’ long hair would be dipped in the ink by the child behind.
The lever on the fountain pen could turn the pen into a squirt gun to shoot ink at others.
Lastly, fountain pens were much more expensive than a pencil. That’s why a fountain fen was a traditional Bar Mitzvah gift in the USA through the 1950s.
When ballpoint pens first came out they were very expensive $20-40. By the early 1960s Bic was manufacturing their blue stick pens to sell for 19 cents in the US. No longer did people stop at the bank or Post Office and refill their fountain pens for free at the checkwriting stand.
Little kids shouldn’t carry penknives (pocket knives) to school to sharpen a quill tip on a pen.
Old habits die slowly and most of you and the teachers are far too young to remember these things and new reasons are invented.
Ask you bubbe or zaidy (if they are 75 or older about inkwells and filling fountain pens in school