Whenever two or more school districts have entered into an interdistrict attendance agreement whereby one of the districts shall provide for the adult education classes for all districts included in the agreement, the average daily attendance of adults attending such classes shall, for apportionment purposes, be reported by and credited to the district in which such adult resides. This section shall only apply to those cases in which the school district providing adult education is a unified school district situated in a county of the sixth class and having, for the 1969–1970 school year, a high school average daily attendance of 9,600 or more.