If you want to get nit-picky about it, the term "Constitution class" is used in TOS, on a table-top monitor graphic in "The Trouble with Tribbles". It's not legible, but then the "Starship Class" plaque linked upthread from "Let That Be Your Last Battlefield" isn't really that legible either.
The back-and-forth here in this part of the thread about what "Star Ship Class" means is amusing, because any interstellar vessel, even a warp-capable shuttlecraft, would be, in essence, a "star ship". For all we know, the use of that term in the context of a dedication plaque installed on a ship's bridge would logically be a ceremonial certification that the ship is now a commissioned interstellar vessel, having been fully outfitted for flight for the very first time.
The idea of a "Starship Class" being different from a "Destroyer Class" is certainly interesting, but wouldn't that imply that the U.S.S. Saladin would not, officially, be called a starship?
I don't see how this makes any sense.