Westchester magazine does a good job on the rankings although the order of them is basically by median income of the area which seems dumb to me. They give you per cent of seniors going to 4/2 yr colleges, avg SATs, avg expenditures, etc etc. Unfortunately there isn't really a magic bullet so you have to look at the data and decide what categories are important to you.
I'm not sure asking the schools would be helpful. I live in the Greenburgh district and the literature they put out sounds rosy enough, then you look at the stats and like 30% of seniors go onto college and they spend more per student than almost all nearby districts. They don't come out and say "our school sucks" even if its true. Generally the top schools are were the most expensive houses are, Scarsdale is usually top of any list, Bronxville is also etc. The bad districts are where you'd think they are, all of Yonkers basically, New Rochelle, Mount Vernon etc.