County fairgrounds are normally owned by the county, along with the buildings. So you ought not to enter the buildings, and if the grounds are within 1000' of school grounds, you'd be in jeopardy.
Now as far as I can tell, the Walworth County Fairgrounds are owned by the Walworth County Agricultural Society, Inc. which founded a charitable organization, Walworth County Fair Foundation Inc. These may be entirely private organizations, making the grounds and buildings all private. I tried to contact the Agricultural Center to verify this, but there was no answer. You can try too: 262-741-3228