BLUF: you may hold your NJMS acceptance and all waitlist slots as long as you want.
By April 30th you may only hold one acceptance, though you may hold as many waitlist slots as you wish. I'd recommend dropping to one school by the end of the week before in order to give schools enough time to accurately report your decision to AAMC.
If, after May 1st, you are accepted off the waitlist at a school that you would rather attend, you may accept that slot and then drop your current acceptance at NJMS. Basically, after May 1st you may only hold 1 acceptance at a time.
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