Green’s Dictionary of Slang

(US campus) a student who is preoccupied with studies.

1981		Eble Campus Sl. Mar. 6: spadet [...] refers to any student, but particularly a science major.
1996		Eble Sl. and Sociability 38: Although blends are fashionable in the marketplace, college slang makes little use of this process of word formation, with only about a dozen examples collected since 1972: [...] spadet (space + cadet) ‘student preoccupied with studies’.