By college, I understand as a school or a university. How to make sense of the College in the term 'Electoral College'?
Etymologically, a 'college' is a group of people 'chosen together' to form a corporate body—the same Latin terms for with and choose lie behind 'colleague'. Thus we have a 'college' of electors, persons chosen to be electors; in the Roman Catholic Church a 'college' of cardinals, persons chosen to be bishops of the first order; and in Great Britain the Royal 'College' of Physicians, persons chosen to be members of the body governing medical practice.
It was only with the rise of the English universities of Oxford and Cambridge, which are federations of 'colleges' of persons chosen to receive learned degrees, that the word 'college' came to denote primarily an educational institution.
The first (earliest) meaning of "College" in the OED (the Oxford English Dictionary) is " 1. An organized society of persons performing certain common functions and possessing special rights and privileges; a body of colleagues, a guild, fellowship, association".
This meaning is now obsolete except in a few specific cases, eg the College of Cardinals (who elect the Pope) and the Electoral College in the USA. Otherwise it has been ousted by the later meaning that you mention.