Assuming your availability date is say six months out, you could accept your first offer immediately (and probably should). Then if you get a better offer later, you can change your mind.
But I'd still keep the 2nd choice on the burner until the last minute, the earlier of start date at first choice or maybe a week before class. Then politely decline the 2nd choice to due to "personal circumstances". You never know what will change in this industry.