The gap in educational attainment between economically disadvantaged pupils and their peers has long been a priority for successive governments. Interventions including the London Challenge, the pupil premium and, more recently, opportunity areas have all sought to address this issue - and rightly so. As a report published today by the Education Policy Institute shows, the disadvantage gap by the end of secondary school is equivalent to 19 months. And while it is narrowing, we estimate that it will take another 50 years before disadvantaged children no longer fall behind their peers during school.

Nevertheless, we might well give ourselves a pat on the back because we are at least moving in the right direction. But one of the primary aims of the Education Policy Institute is to keep interrogating the data, because we can’t just rely on the headlines to tell us how well the education system is serving its most vulnerable pupils.

While the gap is closing for disadvantaged pupils (those eligible for the pupil premium and included in the Department for Education’s performance tables), it is widening for the most persistently disadvantaged (those who have been eligible for free school meals for 80 per cent of their school lives or longer). In fact, persistently disadvantaged pupils are now over two years behind non-disadvantaged pupils. These figures are not captured in any national data or performance measures.

We also know that certain groups of pupils have lower average performance than other groups by the end of secondary school. But what we don’t necessarily know, and what our report highlights, is why some groups of pupils fall further behind their peers throughout school, while others catch up and, indeed, overtake their peers.

Struggling to support SEND pupils

For example, pupils with special educational needs and disability are disproportionately represented amongst the lowest attainers. This isn’t necessarily surprising but our analysis also shows that even the higher-attaining pupils with SEND fall behind as they progress through school. So, not only are they starting school with lower than average attainment, where they are doing well, we are struggling to support them to maintain their attainment levels.

Of course, the type of SEND will be a factor here. But are we looking critically at whether and how well the system is serving these pupils? Could late identification, geographical disparities in identification and variation in support all be contributing to this suppression of attainment among SEND pupils?

Similarly, we know that Black Caribbean pupils are not, generally, amongst the higher-attaining ethnic groups. But what is striking from our report is that these pupils start off doing well but, again, gradually slip down the distribution as they head towards their GCSEs.

Conversely, pupils for whom English is an additional language (EAL) tend to have lower attainment at the start of primary school but then catch and, indeed overtake, their peers by the end of secondary. The phenomena of EAL pupils has caused some to attribute it to the success of London schools, given the diverse ethnic make-up of the capital. While there might be some truth in this, the positive story of EAL pupils risks us neglecting those EAL pupils who aren’t making such fantastic progress. Look closer at the data and it becomes clear that this is not a homogeneous group at all. As a colleague always reminds me, the EAL “label” could refer to the daughter of a French diplomat or a refugee boy seeking asylum from Syria. Again, these are important nuances which are not apparent from the national, published data.

I learn something new from each report we publish. The lesson from this report was clear to me: we shouldn’t look at education challenges in a binary way. It’s not just economically disadvantaged children whom the system often fails. There are thousands of others, up and down the country, whose potential is being stemmed somewhere between the ages of 5 and 16 for a multitude of reasons. We need to keep asking ourselves why.

Natalie Perera is executive director at the Education Policy Institute, and is on Twitter at @natalieperera1

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