Although there are references to a school in Sandbach as early as 1606, the Grammar School was founded in 1677.
“What is known as the GRAMMAR SCHOOL was, however, started about the year 1677, in which year, by indenture dated 28th September, Richard Lea, of Sandbach, assigned to William Hayes, Vicar of Sandbach, Charles Mainwaring, John Acton, Francis Wells, John Furnivall, Richard Lowndes, John Stonier, Randle Rode, and George Twemlow, a part of a field in Sandbach, whereon a building, intended for a school-house, had been erected.” – The history of the ancient parish of Sandbach, co. Chester. Including the two chapelries of Holmes chapel and Goostry. From original records, by Earwaker, John Parsons, 1847-1895 (online here)
A plaque on the side of the school pavilion suggests the school was founded on July 27, 1677: