1869 Third-former . [ see third a. 5] 
1927 J. Elder Thomasina Toddy xi,Sixth and Fifth formers who were in the Elevens. 
Ibid. xii,They gained on the Sixth-formers' boat. 
1928 Daily Express 31 Aug. 15/2 One of them inquired of a small fourth-former whether he had seen any suspicious characters about. 
1960 D. Potter Glittering Coffin vi. 82The choice of grammar school sixth-formers would be Oxford or Cambridge first. 
1969 Listener 20 Feb. 252/1 A more likable, fourth-former-to-fourth-former note. 
ORIGIN: from form  noun -er 1