Library of Congress is a bit pricey. I eat there most every day. (I get prefered pricing)
Better food and prices in Longworth House Office Building Cafeteria. I definately recommend Longworth’s cafeterai.

The Shrine Cafteria is a good one too. And they accomodate large groups well. I’ve seen them handle multiple bus loads of both teeens and adults. Plus, the Shrine is a great place to visit. Very different, and more ornate than the National Cathedral.

For school groups, I like the eateries at Union Station and the Old Post Office Pavillion.
When I was a teacher, I took kids to the Old Post Office. They loved it. Both are a sort of food court, rather than a cafeteria like Longworth and the Shrine.

Some of the other city cafeterias are very small for large groups. They’re really lunch buffet places, and seating is cramped during business lunchtime (12-1:30) and you may not be able to sit eveeryone if you have more than one charter bus load.

Note: the Library of Congress Cafeteria opens to the public at 12:30pm for Lunch, giving employees a jump on the tourists.