Setting aside the ethical issues involved for a moment…
Does the college involved notify the high school? I know that some colleges requiire that the high school sign off on an ED application. Unless you turn down an ED acceptance when you get the fin aid package, you are committed to attend the school. The high school agrees not to send your transcripts to other colleges UNLESS you notify the college you are NOT accepting the offer.
Before thinking of accepting the offer and then waiting to see the outcome of other applications, make sure you high school isn’t bound by the agreement. (It may have to “sign off” that it won’t send your final high school transcript to any other colleges unless your S turns down the offer.) it is not unheard of for this to happen.