Wow, thanks for all the responses guys.
johnleee is not connected with the school, I dunno why he mentioned Premium Max except as a possibility.
DrBC: yes, it's St. Columba's, how did you ever guess?
We're about 250m from the exchange as the crow flies, so the copper run is probably closer to double that.
I checked our line last night, we had 10db attenuation and 9.5db margin. I have no idea if that's good or bad.
There are 300+ computers in the school, but concurrent users probably won't exceed 100. The issue is we run an Exchange server, we use a RDP-based management service (unbelievably slow with just a few users) and there's talk about giving all staff VPN access all of which will require MUCH more upstream bandwidth.
At the moment I'm leaning towards bonded ADSL2+ with Annex M as a strong possibility.
What's the limit on bondable lines? Does it vary by ISP? Can we bond 4x24mb lines?
I have no idea if our budget will stretch to leased lines. Need to figure out what solution offers the best value.
Again, thanks for the responses so far. Very helpful.