Green’s Dictionary of Slang

(UK juv.) a (public) school punishment, usu. writing ‘lines’; also attrib.

1898		Kipling ‘In Ambush’ in Complete Stalky & Co. (1987) 53: No gating. No impots. No beastly questions. All settled.
1900		Boy’s Own Paper 24 Nov. 115: He’d be frightfully strict, and stick you down for a big impot if you moved a finger.
1911		Gem 4 Nov. 11: Oh, blow your impot!
1917		Kipling ‘Regulus’ in Complete Stalky & Co. (1987) 168: Impot from the Head, sir, for that mouse-business this morning.
1929		Kipling ‘Satisfaction of a Gentleman’ in Complete Stalky & Co. (1987) 257: You want a sheet of impot-paper?
1949		Aberdeen Jrnl 13 Aug. 3/5: Parents Did Son’s ‘Impot’ The Head was Peeved [...] ‘Impots must be written by the boy himself’.