Hi Britney,
Wow! What a fun day! I've never planned an outing for that many people, but my daughters have been on field trips to Walt Disney World with large groups. If I recall correctly, they were usually given a gift card to purchase lunch on their own by the organizers. Or they had to bring cash to purchase their own lunch at any quick service venue they chose.
If you are hoping to have everyone together at the meal, I suggest you call Disney Dining and see what they can do for you. The number is (407) 939-1947. In thinking about the Epcot restaurants that take reservations, my instinct is that the
Biergarten buffet in the Germany pavilion would be the one most likely to have room for that many all at once. But the cast members at the reservations line can help you with other suggestions.
I also would make the call ASAP. Reservations open 180 days in advance, and your chances are going to be better if you call now.
If you are thinking quick service, I'd suggest
Sunshine Seasons in the Land pavilion. I believe it has the most seating of any of the quick service restaurants. It can be pretty busy around noon, so you might want to schedule a late lunch or early dinner if you want everyone to be able to get a seat near each other.
Good luck! Please let us know if we can answer any other questions for you as you put together your trip. Enjoy!
Wilma