All of the above are interesting, and I'm a fan of Clint being self-educated - early on or later in life - or self-motivated, as taking every opportunity for education in the army, SHIELD etc.

But I'll also throw in here - disclaimer: based on reading about circus life, generally UK based but some USA, and across various time periods - at some point there came into effect laws about education for child performers/children in circus or carnivals/Traveller's children. There was an arrangement where the kids attended whatever school was nearest at the time, work was assigned just followed you around. (I want to say this was really the 30s-60s UK?) There's also kids who were educated on the road, kind of like home schooling, and in some circus or carvials with lots of kids they'd even have their own kind of 'school' set up with an assigned/hired teacher. So it's possible Clint was made to go to various schools as they travelled - maybe not, because he was a runaway, but maybe yes to throw off any suspicion and keep the law away. Or they could have been a kind of Carnival group school, with a hired teacher - again, even if they were an awful pack of people they might do this to keep the outsiders and law folk off their backs - 'yes, all our kids are getting an education, here's the paperwork, 'kay, thanks, bye'.

I'm more of a 'show me all the different ways this could go' than a fanon person - hence, probably, my love for AUs *grins* - so there's a few more suggestions for the ideas pot.