$320 a day should be plenty if some meals will be offsite. I have taken groups of teenagers to Disney several times, and I tell them to bring enough money to eat lunch in the parks each day(counter service, @$15 per person), plus $5 for snacks, for a total of $20 per person. We have breakfast in the room almost every day, with the exception of Chef Mickey's or 1900 Park Fare, which we budget separately for as well. Offsite dinner is usally pizza to go
($40 for 4 large pizzas), Olive Garden, Macaroni Grill, and other similar chains. Earl of Sandwich at Downtown Disney is great, and costs $8-10 a person. Wolfgang Puck Express is good also, and is reasonable. It rarely ever works out for us to add the dining plan, unless it's free of course!