Router inserted inside current network?

NO.. you can not assign different IP to wifi devices.

you will need WSG series access point ...
http://interface.netgear-forum.com/WAG302/start.htm

Trust me - you really don't want to do that - or at least you don't want to do it that way.

If you "insert" a router into a network, it splits the network into two networks, and unless you know what you're doing, you can end up with a lot of unexpected & undesirable issues.

If I understand you - you want to use the DG834GT to provide wireless access to the school network, without providing internet access - is that correct?

If it is - what you should ideally use is a wireless access point - among other things it can provide better security. Assuming that you're working on a limited budget, you can configure the DG834GT to act as an access point, but this will not provide the enhanced security that a "purpose designed" access point will.

The following instructions do not use the DG834GT's DHCP server, but assume that the 2008R2 server has it's DHCP server enabled.

Connect a PC to one of the DG834GT's LAN ports and open a browser window to the router's admin page - go to the LAN setup and disable the DHCP server, and then change the LAN ip address to an address within the 2008R2 network range - make sure it is outside of the DHCP server's lease pool - if the DHCP server has been configured with an exclusion range, select an unused address within the exclusion range.

Connect the DG834GT to the 2008R2 network through one of it's LAN ports - do NOT use the WAN port - now reconnect to the router and open a browser window to the ip address that you gave it & configure the wireless security.