I tend to agree with UK Taxpayer. If you already have a PBX system in place that provides most of this anyway, why would you want to dump it and spend all that money for a new one. Save the money, get yourself a VoIP Gateway. Work with the other schools to do the same and you will save money on calls without spending the money on a new system. Also, as UK Taxpayer stated, you will need to have someone trained to manage the new system and this will cost money.
Mike