1852
The first (school) building was lengthened twenty feet and the front approach changed.
The regionGrade A · 1 independent source
The evidence
“In 1852 the first building was lengthened twenty feet and the front approach changed. The new brick building on the hill, northeast corner of Church and Bushnell streets, was dedicated July 6, 1862, and seats with the galleries 1,200.”
1908-brown-rock-county/13-beloit-churches.md, Vol. 1, PDF p. 288 / book p. 264
Brown, William Fiske, editor. Rock County, Wisconsin: A New History of Its Cities, Villages, Towns, Citizens and Varied Interests, from the Earliest Times, Up to Date. Chicago: C. F. Cooper & Co., 1908. 2 vols. archive.org/details/rockcountywiscon01brow, archive.org/details/rockcountywiscon02brow.
Grade A: Found on a page of one of our sources