Sonic's Corn Dogs are normally priced at $1 or more (varying with location).
If you've never heard of a Corn Dog before, it's a hot dog on a stick, dipped in a corn meal batter and deep-fried. It also happens to appear on many a school's lunch menu (or at least it did when I was a kid), which makes it an odd choice to celebrate the end of school with.
If the promotion sounds familiar, Sonic offered the same deal last year and also trots it out for Halloween.