Hi ndavis, i had a similar situation to the one you describe forced on me a couple of years ago.
we had a 198.168.0.0/21 range and hampshire gave us 10.207.x.x/21.
we also wanted an ISA server and this is what we have now. Anyway this is our setup now as below.
10.x.x.x/21 192.168.x.x/21
Hampshire feed-------------(eth0 on isa)ISA Server(eth1 on isa)--------------------
LEA Your School
use the lea ip range on the incoming network card of the isa server and your current range on the network card that is internal.
If you have any printers that are used by SAPS(Finance) give them an LEA IP on the external cards and map it to the internal ip.
I hope this has helped and not made things worse.
David
