There are several foods I miss:
Oddly enough, I miss the pizza that was served in my middle school cafeteria-a big rectangle covered with cheese and a little chunks of pepperoni.
Also from middle school, the chocolate chip cookies-the size of a saucer, always hot from the oven and still almost-but-not-quite done, very melty and gooey. Yum!
Speaking of school food -- I hated a lot of the food served at school when I was in the LA Unified School District. However, the one thing I always really liked and have found myself missing for years is the burritos they used to serve.
When I went to high school in San Diego, they had weird burgers where, instead of them being two split buns with meat in between, the meat was completely encased in the dough then baked. I really liked their BBQ beef burger (sandwich?) and the cheeseburger.
Anyone remember when Oreo had it's own ice cream "line" (all cookies and cream ice creams, just the ice cream flavors were different). I used to love their chocolate ice cream with Oreo cookie bits (I think they are selling it now again and touting it as a "new" item, which I find really odd since it was around about 20 years ago). There was also vanilla and mint, I believe (I think there were other flavors, but I didn't eat them).