Traffic will be heavier during the day, yes, as there will be a large amount of Guests. The Resorts are usually filled to capacity during Grad Nights. The vast majority of Grad Night activities take place at the Magic Kingdom, and they generally run from midnight (after they've closed the park to regular Guests) until 5 AM. You may, however, see large groups being led to and from various attractions later in the afternoon. As part of Grad Night, some schools opt to have an event (e.g. dinner in the Rotunda of the American Adventure and a private IllumiNations viewing) before heading off to the Magic Kingdom.