The institution of widespread public education that looks anything like what we have now isn’t something I’d describe as having meaningfully existed for hundreds of years, let alone as having been stagnant that whole time.
https://en.wikipedia.org/wiki/Prussian_education_system
If you mean the university and college levels, they have interesting differences from the 18th century, but are recognizably similar in regards to basically everything besides cost and curriculum differences we'd expect due to changes in societal needs, technical advancements, and changing interests.
If we’re going back farther than that, then I’m really having a hard time seeing where the stagnation comes in—I don’t agree with that even past that point (gifted education alone is only just now starting to develop into something halfway useful, and that’s a very recent change in just one small part of primary and secondary education) but if we’re going farther back, then… what?
Letter grades are an 1897 invention and only at one college to start with: https://studysoup.com/blog/uncategorized/history-of-the-lett...