A new project to allow schools to compare their data has uncovered a “poignant” spike in the number of poor pupils who miss school on Christmas Jumper Day.

The early finding from a union-led project in North West England led some heads to stop celebrating the day in their schools.

The initiative from the Association of School and College Leaders, which has a working title of the Open Data Project, got underway last autumn, and allows clusters of schools to combine their data to look for trends.

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Duncan Baldwin, deputy policy director of ASCL, outlined one trend that was identified when groups of schools pooled their attendance data.

“We were looking at disadvantaged pupils particularly, and there was this dip on 14 December in several of the schools”, he said.

“We were looking at each other, saying ‘what’s special on 14 December? No idea’.

“Look on Google and, lo-and-behold, it’s Christmas Jumper Day.

“What we’ve identified, which is bleedin’ obvious if you think about it, is that days that are ‘fun’ and for good causes are not necessarily fun at all if you don’t have a Christmas jumper.

“The net result is that a noticeable proportion of disadvantaged youngsters didn’t come to school that day.”

He described the finding as “poignant”, and added: “We had heads immediately ringing their schools saying ‘get it off the calendar’.”

Mr Baldwin told Tes that another finding, which will be investigated further, was that some disadvantaged pupils miss school on their birthdays.

“There may be several reasons,” he said. “For example, if they go to school and other kids know it’s their birthday and say ‘what did you get for your birthday’ it may be a very difficult question to answer, or maybe families say ‘it’s your birthday, it’s a special day, so have the day off school’.”

He said that once such a trend was identified, schools could take action to address it, such as using pupil premium money to buy pupils a cupcake on their birthday to give them something to celebrate.

The project also aims to investigate concerns from Tameside secondary heads that a two-week Whitsun holiday taken by local primary schools reduces attendance of older siblings, who might go on family holidays, or have to act as carers.

Mr Baldwin said the project uses existing data held by schools and does not increase their workload.

He added: “The ambition is to say you’ve got all this data about your pupils - what happens if we were able to aggregate, share or benchmark that data in ways that haven’t been possible and visualise it and use modern data tools on it.”

The project currently involves 12-14 secondary schools in Tameside, Lancashire, Liverpool and Manchester, but ASCL wants to expand it, prioritising “strategically important” locations, such as the DfE’s Opportunity Areas which have low levels of social mobility.

Mr Baldwin said that schools interested in joining the project should contact ASCL.