@RonJohn I've heard both, as a younger-than-you also native English speaker. I do agree "E for effort" is more common (and the one that I most commonly use) but I have heard both. I've never (well, rarely, not never) heard it used for academics though.

@SethR I had that grading system on my report card sometimes but IIRC my assignment grades were always letter grades or percentages.

"Yet" is kind of archaic and sounds odd in today's English. But I suppose it would work.

@J... My understanding of "E for effort" is that it is similar to "A for effort" except ironically using the letter "E" as a near-failing grade to imply the failure of the endeavour overall, but also because the word "effort" starts with E; you are giving an "E for effort" to recognize the effort while also ironically emphasizing the failure of the endevour.

Neither of the sentences provided make sense/are idiomatic. Can you provide a clearer description of what you intend those phrases to mean (in context) so we can provide you with an answer as to how to construct them, and explain what is not idiomatic about the ones provided?