That was always the way serving was done at my school when I was growing up: whatever was on the menu was served, on every single tray. If you didn't want it you either gave it away or tossed it, but the lunch ladies didn't have time to be asking each child whether or not they wanted something; they just handed you the tray. The average large school kitchen that served the foods that Oliver advocates would probably have no choice but to pre-plate and use warmers, so yes, you would have to cook enough potatoes to serve every child.
I can absolutely see why some food service personnel would be resentful of Jamie Oliver's style. He can be very patronising, and he has a really ridiculous tendency to think that the specialty equipment needed to produce in quantity the kind of food he advocates is just normal school kitchen stuff, when it isn't normal and is quite expensive; even the average hotel kitchen would most likely only have half of it. Most school kitchens that I have used (in a volunteer capacity) over the past 10 years only had one piece of specialty equipment other than a 6-burner stove and a large-capacity sink; an industrial mixer for baking. A lot of older schools don't even have walk-ins.
How do 3 people peel 1400 potatoes in 90 minutes while also prepping the rest of the meal? There is only one way: use a commercial potato rumbler. They cost around $1000, are about the size of a home dishwasher and can handle a bit less than 30 lbs. of potatoes per cycle. 1400 potatoes is going to be probably 600 lbs.; you do the math. Also, rumblers don't tend to handle white potatoes well; they do best with red, which really are not that good for mashing.
I know that around here, the total amount of money that most of the schools can spend on meal production is $2.80 per child. Most budget-conscious folks would find it easy to spend that much and serve fresh food at home, and think it should be easily doable when buying in bulk. So it would be, if the money only had to go to buy food. It doesn't though: it also has to pay for the workers' salaries and health insurance, FICA contributions, workers comp. insurance, software and office supplies, equipment costs and maintenance, paper supplies, and sometimes even the utilities for the kitchen. It takes years of practice to correctly and consistently hit the FDA requirements for school lunch values with fresh foods; the rules are written in a way that discourages their use because they are difficult to quantify. Also, remember that in a lot of districts 2 meals per day are being served, not just one.
I'm not saying that Oliver has a bad idea, just that restaurant techniques are generally out of reach for the average public school system as funding now stands. Most of the time they are going to need more staff and a lot more equipment to pull it off, and that leaves less money to purchase all of that fresh food. With most schools currently dealing with cuts in curriculum due to tax shortfalls, this is not likely to be an area where big ideas are going to find fertile ground.