Never let your schooling interfere with your education.
Variants: Don't let your son's/boy's schooling interfere too much with his education. I have never let my schooling interfere with my education.
Not directly traceable to Twain; first attributed to him in early 1900s in latter form, as in Outing: sport, adventure, travel, fiction, Volume 50, 1907, ed. Caspar Whitney, Albert Britt.
http://en.wikiquote.org/wiki/Mark_Twain
Both great quotes (but it should matter who originally said it -- I know I wouldn't like it if something I said was attributed to another, especially if that other person is famous and there is no way I will ever get the proper recognition for my comment/speech).