Can we see a floor plan?
I can see what you are trying to do and why you are trying to do it.
Unfortunatly some school layouts just mean that it is not possible to get away with a single cabinet to serve the whole school. If you've only got 4 buildings you don't sound in too bad a position. You could get away with 1 x main cab in server room and 3 x satelite cabs linked via 10 gig fibre. That would be ideal performance wise and shoudn't cost too much.
We have 13 buildings and as the infrastructure has been slowly addded to over the years - cabinets and fibre everywhere. I would like to rationalise the cabinets as much as possible but realistically there isn't much rationalisation that can be done without taking a bulldozer to the site and starting from scratch.
Replacing all cabs and re-wiring with cat 6 would be a luxury for most schools IMHO. You have to ask - how much value will you get from that estimated 100k spend. How much will teaching and learning quality improve via the use of that 100k/ The answer to both questions is not much / barely any. As such - I'd have to make DAMN sure there wasn't any other way that the 100k could be better spent before I committed to the network cab project.
Either way - I am jealous that you are even able to propose such an idea!!
We rewired our entire school this summer. We put in one new network cabinet (for a total of 10 - one on each floor, plus the server room), but kept the current network cabinets in their existing locations. In total we put in approximately 1300 network points, one 8-core fibre run and replaced all network cabinets for a cost of around £60,000. We already had 8-core OM3 fibre runs to all the other network cabinets.
Unfortunately the distances were to great for us to even think of running it all back to a single point, however if we had any spare hidey holes/large cupboards it would have been good to have been able to relocate some cabinets to them. Unfortunately we don't so everything still wall mounted in various classrooms.
This really depends on how populated your cabinets are, but:
1) You can get slimline cabinets that mount kit vertically (switchports are at the bottom / top). Like the Thinline model here: Wall-Mount Enclosed System Table - At Chatsworth Products, Inc.. Assuming you can get sufficient switchport density per cab, that may be an option.
2) The wallspace in broom cupboards / understairs / storerooms is under-used. There's normally one within 30 metres of where you need to provide network access.
Search for TA24 5RH on Google Maps, that'll give you an idea. Only change from the google image is the long hut on the playground is now gone, and in its place 4 classrooms as an extension to the topmost building of the site.
There are no spare cupboards - the cleaners/caretaker have 1 store of their own, and that's a shed.
Ok, so, distance aside (as we would settle on 2 rooms instead, which would cover the site), anything else?
We can't really do 'slimline' cabinets, as each one around the school is at least 12U and we use very deep switches (HP Procurve 2650s or deeper in some cases (PoE models).
The advantage I can think of is the ability to increase the backbone from a couple of 1Gbit fibres to 10Gbit relatively cheaply, and then link the switches with 2 Cat5e cables for 2Gbit connectivity to that. It'd effectively increase the network speed to at least double what it is now without much new equipment.
You have to be careful running copper between different buildings because of the potential earth difference between them, it can cause problems if you're not careful.
I can understand the need to rationalise cabinets as I did the same when I started where I am now - we had cabinets everywhere when I started! Slowly over the last 2 years I've managed to get rid of 3 of them so it's a bit more sensible now. However I would never consider getting rid of all of them back to a central location unless that location had proper decent easy to follow cable trays/trunking going out to all the locations necessary, otherwise it makes adding additional network points very difficult and very expensive. The best I would hope for would be 1 cabinet in each building and a good multi core fibre link back to the main comms cabinet, but having all the copper connections running back to a cabinet in another building is an un-wise thing to do in my opinion.
If you go for underground connections - from hard experience make sure that the installers make both ends of the conduits they bury completely rat-proof. For some reason our local rats think that fibre-optic stuff is as tasty as anything else in the known universe - we had two breaks before we had things totally protected!
Keep your satellite cabs, gives you some resilience in case you have issues around the school site. One of our biggest sites has the main server room and has a number of satellite 42U cabs, can often make fault finding a little easier too with switches.
As said previously, length of copper is 100m max, but this can also be reduced by interference from other stuff along the route that's near the cable. I presume you would be following paths of existing cabling, if so, either buy or rent a fluke tester that can do certification testing and check for current levels of interferance on the cables. You then need to determine a main run route to your proposed cab location, pull a cable through and test. All these tests need to be done when the school is in use and the majority of electrical equipment is switched on, especially lights. If you're not getting results well within the upper limits for noise then you need to reconsider or find new routes through and re-test.
Having thought about this a bit more, we could feasibly do it for around £60k, as many of the network sockets around school are either in ridiculous places or no longer there (due to classrooms changing purpose and the like) etc... So, we'd be able to actually design the system around exactly what we want and where. It'd also allow us to put proper cable management in everywhere and follow sane paths back to the cabinet (cable baskets, reduce the massive number of bits of trunking everywhere down to single strips and dado trunking etc...). It'd also allow us to reduce our switching from 15 different devices down to 2 cabinet switches (a 5412zl and a 5406zl), which actually reduces power usage by about 40% or so too.
Now, the question remains - putting cable in ducting underground, what are the pitfalls of this? Someone mentioned grounding issues?
I think that's 2 do with if you are using cat5/6 linking between buildings or from pc to main cabinet, if the building are on different phases you could induce ground issues.
The problem is earth potential.Data is sent at 5v to earth. If a different earth source is used in different building there could be a voltage difference between the two earths.This would make the data invisible. I have seen it when I was in the RAF, stopped a radar from being useful. Until we bonded the earths to a common. We had a difference of 30v.