The UK doesn't have Free Speech in the US sense, nor a Constitution (with a capital C). I don't think there's a right to photography of private (albeit publicly owned) property, especially a school.
If you read my comment above, she is not photographing their private property (the school) but her own private property (the bought lunch). They say they can impose any rules they like as a condition of entrance on their property, even though it is not a voluntary entrance. Her parents may have a choice to send her to a non-council school but she has no choice.
As to the UK's non-existent constitution: the point is that in most countries a stronger parliamentary majority is required to change the constitution, whereas in the UK any shaky government, that just happened to muster a majority of one on a rainy day, can abolish the Magna Carta, for instance.
