My school has multiple buildings but we still eat indoors (were still discussing this, right?)
At the high school I went to for half of freshy year, junior and senior year we had multiple buildings, but come lunch time they didn't care where the students went. A lot of people went in the cafeteria but most just chilled outside, or in a teachers classroom. During senior year I was always chilling during lunch in random teachers classrooms, or in the JROTC room.