It is an honor system. You can still apply to other schools and even accept other schools'' offers if you insist on doing so. However, rumor has it that if you reject the ED offer, that might impact your high school''s future graduates when they apply to that same school. Hence the consolers usually are very careful about this, and always strongly against doing so, i.e, changing your mind after you get the ED offer. Some Ivy League school has unofficial rules of withholding any offer to the same high school for the next five years if any of that high school''s graduates reject their ED offer.
