A polo shirt is at the smarter end of the t-shirt range. A t-shirt is T shaped. A polo shirt is t shaped, so still the same letter of the alphabet.
DS2 wears polo shirts for school. DS1 wears shirts to be worn with a tie. "School shirt" would mean DS1's uniform and for laundry purposes, I'd just talk about a white wash.
DH tends to favour polo shirts as casual wear.
A sqaure is a rectangle. An oblong is a rectangle. Squares and oblongs are mutually exclusive though 😁