How are recruiting targets for MBB managed with respect to schools?

Your question is a bit hard to interpret and it seems you're asking a few things, but let me attempt to answer.

First, MBB (like any company) recruit where they have historically had the most success. So, they will recruit at school that are both top-ranked and seem to produce candidates that end up integrating well with the company. This is the only priority/concern...alumni do not influence this.

There are strong concentrations of particular schools because 1) Better schools produce better candidates, with a long tail and 2) Regional schools will naturally be better represented (not just because it's logical but because students there are mor elikely to recruit for the local office).

If you studied economics you would understand these forces as the market for lemons (which involves signalling, and asymmetric information)

As such, for Chicago, you better expect that Booth and Kellogg are the best represented.

No. There is absolutely no reservation or quotas. You see these concentrations because some schools have partnerships with specific offices so increases the chances of getting an interview but I can assure you there is no "alumni pressure"

They certainly do not reserve spots for kids from different schools. That would imply a stronger candidate from School A could lose his spot to a weaker candidate from School B. That's no way to build a Tier 1 Consultancy.

What they just might do, is reserve a specific number of interview spots. Say (pre-Covid) a group was going out to interview at School A. They usually want to fill up the day. So they'l interview the 10 strongest candidates. It could be that somebody might get an interview at School A that he or she wouldn't get at School B.

However, this is just nitty gritty. Should not influence your school choice and should not influence how you apply. And nobody can get hired unless they do really well on the interviews, so prepare well!