Green’s Dictionary of Slang

boarding scholar n.

[ironic use of SE]

(UK Und.) a prisoner.

c.1698		B.E. Dict. Canting Crew n.p.: Boarding-scholars, c. Bridewell-birds.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].