I am certain they could have done well, as they were plenty smart. Paul and George got into their school because of their high marks on the entrance tests-- they could not have afforded it otherwise. This means an IQ of at least 115, based on other students who passed the exam with that score (and yes, I have forgotten where I read that!).
However, in not getting along with the educational system, they were exactly like Einstein. From Wikipedia:
Albert clashed with authorities and resented the school regimen. He later wrote that the spirit of learning and creative thought were lost in strict rote learning.
The same entry notes that Einstein flunked the entrance examination to the Swiss Federal Institute of Technology in Zurich at age 16. He did all right with his life despite that setback.
George in particular
hated the school regimen, although he turned into an ardent scholar in his twenties once he was free to pursue his own course. All the Beatles (except Ringo, who never learned properly) were big readers throughout their lives, as more than one biographer has noted.
I'm sincerely glad they succeeded in music, because I think they would have been bitter and miserable at a fulltime job. Some of us can make the adjustment. I don't think John ever would have.