The Department for Education in the UK have just published a report showing the correlation between attendance at school and exam results.
They say that:
At Key Stage 4, pupils who attended school nearly every day in Year 11 (with an attendance rate of 95-100%) were 1.9 times more likely to achieve the Grade 5 in English and Maths GCSE compared to pupils who only attended 90-95% of the time (relating to up to 2 weeks more time in school over the course of a year) and 3 times more likely than persistently absent pupils who only attend 85-90% of the time (relating to 4 - 6 weeks more time in school).
This means missing just 10 days of year 11 reduced the likelihood of achieving grade 5 in English and Maths by around 50%. Pupils who attended school 90-95% of the time were also 1.6 times more likely to achieve Grade 5 in Maths and English GCSE compared to pupils who only attended 85-90% of them time, relating to an additional 2-4 weeks in school.
You can see the whole report here: The link between attendance and attainment in an assessment year