Christ's Hospital is so called because the boys there wear a long blue coat girded at the loins with a leather belt. Some who attend the mathematical school are termed King's boys, and those who constitute the highest class are Grecians.
Founded by Edward VI. in the year of his death. There are several other blue-coat schools in England besides Christ's Hospital.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894