What? Who said being at a boarding school improves your chances if everything else is excluded? You can’t exclude the “everything else.” The everything else is what actually gets you in.
You seem to have no idea what top colleges are looking for. Please do some research. If you truly want to get into a top school, you need to know what they want and SHOW it to them. They can’t just admit every kid who goes to boarding school.
Understand this: your boarding school might get you to the gate, along with grades and test scores. Without the “everything else”, you aren’t going to get through the gate. Certainly not wihtout the grades in the first place.