Schools send out their own packing lists but this will give you an idea and get you started:
http://talk.collegeconfidential.com/prep-school-admissions/311892-the-ultimate-supply-list-for-boarding-school-p1.html
http://talk.collegeconfidential.com/prep-school-parents/1641101-updated-thread-for-finding-bs-stuff-p1.html