I work IT in schools (always have, primary to further education, state and private, 20 years experience):
Yes, almost every classroom has an interactive whiteboard, projector, PC (of some kind), sound system, etc.. The cost isn't huge. About £5000 per classroom, at best? A whole school upgrade would generally be done every 4 years, on average (or 25% a year, etc.)
Now they are old-hat and being replaced with giant touchscreens. Same cost, but no separate wiring (no power + VGA/HMDI into the ceiling, no speaker cabling at all, etc.).
Laptops to the kids is still not common, but I work in a school with 1:1 iPads (i.e. an iPad or Chromebook each) - it is however a private boarding school, but it's not unusual for a state secondary (or Academy) to have such too.
I'd estimate in a school with 1000 kids you'd likely have AT LEAST 50k-100k of IT investment a year. Not including staffing/outsourcing. In-house servers, maybe, hundreds of PCs, dozens of whiteboard/touchscreens, all kinds of nice back-end storage, switching, routing, leased-lines, the works. Just for your average secondary.
Such costs are lost in the noise of any building work. Literally, error-margin afterthoughts - even with cabling costs. The IT department (as in actual techies) may well be funded to the same tune as, say, a maths department, or ICT (i.e. teaching computing) department.
Portacabins are used for a reason - planning permission is hard to come by and expensive. Temporary portacabins are cheap and don't require planning. You often can't expand school sites as you can't justify buying land or building on "school playing fields" (which have special rules in land use). That portacabin is a trick to put more pupils into a school and save you lots of money as a taxpayer. One portacabin likely costs enough to fund an IT department, by the way.
A school building project for, say, 5 permanent classrooms, might cost you upwards of a million pounds to implement. There are strict rules on making profit (even as a private school!), how much you must set aside for emergencies, what financial agreements you can get yourself into. Hence 5 classrooms - so 150 extra kids in a state school, less than 10% growth on your average state - will cost you orders of magnitude more than anything the IT costs for the whole site. It would take you 5-10 years to get the planning permission. In that time, your intake may well have increased by way more than 10%, especially if near a city. The staffing going up 10% will swamp the building costs, which would swamp the IT costs, which are likely cheaper than the "temporary" portacabin solution on its own.
And any decent school IT Manager manages their budget, tests, trials and prototypes like any IT department of a large corporation with 100+ staff would (current staffing ratios in some schools can approach 1 staff :4 pupils if you include all the estates, maintenance, IT, HR, admin, social care, etc. staff).
P.S. The tech is HEAVY. Name another IT place where you would expect every user to log off every hour, move to the other side of the school, log on there and get teaching in less than a couple of minutes? For 1000+ users? With mobile devices, site-wide wifi, etc. Registration is electronic, medical records, assessment (including national exams), the kids and staff have VLE access (basically work-from-home remote-desktops), VPN, almost all marking, commenting, evidence, target tracking, etc. is done on school IT systems, everything from a text home because Johnny was five minutes late and is he playing truant, to site-wide CCTV, access control, remote servers and remote secondary data locations, IP telephony, etc.
The schools of old aren't comparable. The tech is necessary and integrated to everything you do. Even receiving new pupils into your database is an electronic transfer from the local authorities. And yet all the IT costs less than that portacabin, which is really a cheap-out trick because of planning laws.
And, state or private, primary or secondary, the situation is pretty much the same, only the scale of pupil numbers differs, not the ratio of tech.
My pupils and staff created 1 million Google Docs documents in the last 2 years. It cost us £0 to do that (Google Apps is free for schools). Now imagine the supporting systems to facilitate that. Microsoft licensing is my biggest single item expense, and that's charged per Full Time Teaching Employee Equivalent (i.e. if I have only 40 FTE staff, I pay for only 40 copies of Office, Windows, RDP licences, etc.).
Don't blindly knock IT in schools, because it's probably the best value IT you'll ever see and heavily used and integrated into the business. Question what use is being made of it in lessons, sure, but the IT in lessons is almost irrelevant to the overall cost of even a couple of teachers.