And the rates are now among the highest Covid-19 rates in the country among young people.
That’s according to West Sussex director of public health Alison Challenger who sent out warning letters to all secondary school headteachers last week.
She said that the surge in cases was revealed in latest data from November 19.
In her letter, copies of which have been posted on social media, she says: “Rates have risen sharply in children and young peope aged over five, particularly 10-14-year-olds with rates of over 2,000 per 100,000, more than double the national average for this age group.”
She called on headteachers to re-think plans for indoor Christmas events which could lead to large numbers of people mixing together.
She also called on heads to reintroduce the wearing of facemasks in communal areas - and most schools have now made this mandatory from this week.
Ms Challenger said increasing numbers of Covid cases were now also being seen in primary schools.
She said keeping schools open remained a priority but urged school staff to continue to promote regular testing and the vaccination of staff and eligible pupils.
Some schools in West Sussex which have experienced a large outbreak of Covid have already cancelled indoor Christmas events following advice from the UK Health Security Agency, formerly Public Health England.