Green’s Dictionary of Slang

1. one who regularly fails their examinations or recitations.

1896		W.C. Gore Student Sl. in Cohen (1997) 5: flunker n. One who habitually fails in recitations.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 36: flunker, n. One who fails in examination.

2. a teacher who often fails students.

1910

O. Johnson Varmint 14: Mr. Hopkins, alias Lucius Cassius, alias The Roman, master of Latin line and distinguished flunker of boys.