Is boarding school bad for children?

Yes. Unless you are a totally incompetent parent you should invest the TIME to rear your children rather then entrusting them to professionals.

From jimkingwood:

boarding school starting in about the 9th grade can be very beneficial for some children who are ready for independence, challenging curricula, the additional educational and extracurricular opportunities these schools provide, the pride and cohesion that comes with living at the school, life-long friends, networking and getting into the best colleges and careers after graduation. Many boarding schools offer significant financial assistance.