The food court at Union Station is very popular for high school groups, as are the food courts at the various Smithsonian museums. The Hard Rock Cafe near Ford's Theater and the FBI building also packs 'em in. As a general rule, avoid the buffets, especially the pay-by-the-pound ones.

Most of the places listed by the OP are in Williamsburg, where the choice of venues for feeding hordes of teenagers is much more limited.