I think maybe I'd summarize my longer post above like this. I'd only consider sending my DS to Andover if I felt:

1) He was independent: eager and able to be away from home.

2) He was socially mature: able to make friends easily and likely to be happy there.

3) He was emotionally mature, secure in himself and well grounded: so he could resist teen peer pressure with less adult oversight than he'd have at home.