Interesting - thank you! It turns out (via the internet) that Forester's elder brother Hugh attended Christ's Hospital, and that CSF also won a scholarship there, though at the last minute he was sent to a different school for financial reasons. Either way, he would certainly have known of the CH usage through his brother. Hard to say whether that affected his choice of word for Hornblower, or whether he simply meant that Hornblower studied Greek, or whether "Grecian" was a term used at other schools besides Christ's Hospital (though the OED entry suggests perhaps not).