John Sr. joined the service in WWI after finishing high school (mentioned in other episodes especially season 8 "The Diploma"). If this is his 25th high school reunion it would make the year of the episode between 1940 and 1942 if he graduated in 1916-1918 before WWI ended but after US troops started going overseas.
John tells Rachel Stubblefield that he has four boys and two girls. He actually has four sons and three daughters.