Keisuke has been studying here at this estate the whole time. It's what they call homeschooling, basically.
Literal
Keisuke-[honorific-さん] [topic-は], this [honorific-お]-estate [at-で], continuously [honorific-お]-studying-[continuous-polite-しています]. So-called homeschooling [called-という] form is-[polite-ですね].
Polite explanation. お屋敷 ('mansion, estate') uses the honorific prefix お — 屋敷 is a larger, grander house than a typical 家. ホームスクーリング ('homeschooling') is a loanword. いわゆる ('so-called'). ずっと ('continuously, the whole time'). お勉強する uses the お-prefix-ization of 勉強 for polite formality (often used for children's activities).