If Jethro graduated from the 6th Grade in the 2nd season finale, why couldn't he have gone on to the 7th Grade, the 8th Grade, or even high school? I saw him graduate college in a later episode. He believes he is such a scholar, so why didn't he try to further his brain?

That's just the extreme (and completely unbelievable) hillbilly comedy. The Clampetts thought gradiating the 6th grade was a very high level of education, and thought that at least 2 years at each grade was normal.

If I remember right, I believe it was stated in one later episode that Jethro gradgiated 6th grade 3 times :lol:
Also, I believe Granny wanted to further Jethro's edggycation herself in another later episode by performing the first head transplant, with Elly's pet chimpanzie Bessie as the donor, Jethro as the transplant receipient.:lol:

This episode was on TVLand yesterday, oddly enough. When Jethro says he'll be done with "schoolin'" after the graduation, Miss Jane says no, he still needs to go through the 12th grade and then to college. Jethro quickly figures out this will take about twenty more years and isn't happy about that prospect, so he gives up his ambition to be a brain surgeon or an atomic scientist in favor of being a streetcar conductor.

One funny line: "College!?! All I want to be is a brain surgeon!"

This episode was on TVLand yesterday, oddly enough. When Jethro says he'll be done with "schoolin'" after the graduation, Miss Jane says no, he still needs to go through the 12th grade and then to college. Jethro quickly figures out this will take about twenty more years and isn't happy about that prospect, so he gives up his ambition to be a brain surgeon or an atomic scientist in favor of being a streetcar conductor.

One funny line: "College!?! All I want to be is a brain surgeon!"
I don't think it would have really done him any good.