To be honest, I think a lot of applicants (and current students) overestimate the value of alumni networks in their careers. Can it be of help? Sure, if you're a current student and you reach out to someone who graduated say 1-5 years ago (older alums tend to be less helpful to current students). But it's not some secret society or anything where you're bound by blood to help your fellow "alums" out. It's far more casual than that.
The most valuable part of the "network" is your current classmates - your friends and your friends of friends. Not some random person who happened to graduate at the same school you did, but is like 20+ years older than you.
As a current student, you will likely get way more job leads from your fellow classmates (and friends you may have at other b-schools) than you will by calling up alums who are complete strangers. In fact, beyond traditional on campus recruiting, a good chunk of your job leads will come from your classmates/friends who have the ability to make mutual introductions to people they know.
That's why alumni networks aren't as important as your classmates - when it comes to schools, you want to choose the ones where the environment is best suited to your personality and needs - in plain English, the place where you feel you can make the strongest friendships (not necessarily the "most" friends, but the "strongest" friendships). And also, it's because these friendships and relationships will hopefully endure beyond your time in school.