Complex sentence: The area is home to two of the three prestigious "Hill Schools" schools, the Horace Mann School, and the Ethical Culture Fieldston School; the third, Riverdale Country School, lies just outside Fieldston to the north.
Split into simpler sentences: The area is home to two private schools: the Horace Mann School, and the Ethical Culture Fieldston School. The Riverdale Country School, the third of the three prestigious "Hill Schools" is just outside Fieldston to the north.