Have you add recently a larger work load ?
does it most of the traffic come wirelessly, I suguest a process of switching of part of the network, trying to isolate where the network load is coming from. there may be a rogue element in your network doing bad things malware.
as for the work load on the server, I do not know xserve or mac os x server, but i do know networks and administration. Yes you can split you clients from staff and admin serpate. These can be all administored from seperate machines. That we be a good move. Next move the ldap authuucation for the clients(pupils) to a server by it's self. this leaves the xserve as a file server. should free up more resources. If you can put mac os x server on other computers other than xserve, these I would do and use this to authenicate the accounts. I would then set up all the directories on the xserve, it may be good to have 2 xserves, one for pupils one for staff/admin. This way if the pupil server get hit, then the staff are not effected. also reduced everybody on one sever.
3 changes I would do,
-move ldap to seperate server (like an emac with mac os x on it)
-split the xserve into two groups, for files sharing pupils on one staff on the other. (need a second xserve or a high end powermac with mac os x server on it, this second is for staff/admin).
-search for where the traffic is coming from. It could be over load on ther server or a rouge element.
using activity monitor on the xserve check to see network acivity and processor load, if the processors are over 80% used this is a good indication that you server is swamped, if the network is maxed out then you network interface is swamped.
if it is the network interface, and the process are low, then a second interface on the system will help take the load.
my guess is that most of your traffic is wireless, this has a problem in that two many users will slow it down, wireless is not good to use with a remote file server, for to many users, the amount of data is hugh and it will be a major bottle neck. You might want to seperate you wireless network into zones, to reduce the number of users on one hub. also you might want to make sure the backbone from each wireless base station is at least it own 100 base T. Do you have a fast switch at the hub, with 1000 base T to the servers.
Hope this helps