I rather suspect that the practice of children wearing hijabs is not because of concerns that they will be sexualised at that age per se but in order to inculcate the practice from a young age so that once they reach sexual maturity, they are habituated to doing so. One might be tempted to infer from this that there might be a concern that if they were wait until the girls were older, they would refuse to do so. It is rather disturbing that the article reads as though the parents think puberty is determined by age rather than physical development, but this may be down to bad reporting.
The school is clearly of a religious character - viewing their admissions policy, it appears that many places are reserved for children of Greek Orthodox parents (and in decreasing preference Catholic, other Christian and finally other faiths (always good to see the children of non-religious parents being discriminated against by state run schools, btw)). One has to ask whether there are similar prohibitions against the wearing of Christian artefacts (I would imagine jewellery is banned, so probably not much in evidence) or whether a Sikh child would be barred from wearing a turban?
Having read the school's uniform policy on its website (http://www.st-cyprians.croydon.sch.uk/page14.html) there is nothing that states a hijab is banned, and it's Race Equality and Cultural Diversity policy explicitly mentions:
"The school is opposed to all forms of racism and xenophobia, including those that are directed towards religious groups and communities, for example Islamophobia and against travellers, refuges and asylum seekers" -
https://docs.google.com/viewer?url=http://www.connectedup.com/stcyprians/policies/RACEEQUALITY.pdf
While I agree that the wearing of the Hijab is not mandated, it may be that there is iniquity in singling out this one child (or one faith), and if so, there's a case to answer.