MX record config for sub-domains...

[OSquires]

In my public DNS I've got 24 sub-domains (school locations) configured 
inside the top-level zone file.  The system's mail server, etc is 
configured in that same zone file.  Now I've got a situation where I 
need to provide an MX record (e-mail services) for a few of these 
sub-domains.  I assume if I break out all the sub-domains into 
individual zone files that would work.  Is there a way to configure BIND 
(v.9.2.1) so that I only need to break out the sub-domains that I want 
to add e-mail services to?

Hope this makes sense - thanks,

Owen