Sean Macnamara was put on "the oblong table" for low-ability pupils when he was still in reception.
No-one told Sean and his friend Billy what being "an oblong" meant - but they knew.
Smart lads like Matthew and Paul (Sean still remembers their names) were on higher-ability tables.
Sean believes the oblong-table pupils were set up to fail from the outset.
"We just used to mess around and be really juvenile and we didn't achieve anywhere near our potential.
"And I don't think that's because we were of low ability.
"Lots of us are now successful in different areas but in school it was almost like we were written off."
Sean is now Mr Macnamara, deputy head of a primary school in Lewisham, south east London.
He says he got there almost by accident and certainly not because of his early experiences in school.
Struggling in the bottom set, Sean hated reading, and when a teacher asked him if he enjoyed taking a book home to read each week, he said: "No".
Rather than force it, she decided to stop the books until Sean was ready - but the effect was to reinforce his negative view of himself.
"Even at that age you're aware of what's going on around you.
"Every week everyone else would change their reading books but I didn't need to change mine because I didn't have one.
"I think it's very much like a snowball.
"It starts small. It doesn't have much of an impact but over time it grows and perpetuates until you find yourself in Year 10, in the bottom set, with a predicted grade of a D for English."
Sean did not get good grades at GCSE but excelled at creative subjects and did a degree in product design.
Afterwards he decided not to go into industry, opting instead for a postgraduate certificate in primary education.
After a couple of years he decided teaching would be a lifelong career.
"So I might as well try and learn as much about the craft as I can and be the best that I can be."
He went part-time and embarked on an postgraduate degree in education at University College London's Institute of Education but he was plagued by insecurity about his own abilities.
"I felt I didn't fit in. I didn't belong."
Looking back, he says he can trace it all back to the oblong table.
In his dissertation he explored the impact of ability grouping on pupils' wellbeing and attainment and his research shed new light on his own story.
"It opened a whole world I didn't know existed."
The bottom group
The previous academic papers he read to research his dissertation suggested that his own bad experiences of ability grouping were far from unusual.
Study after study suggested that being in the bottom group actually deters learning.
And when he came to do his own research with pupils in the school he was then teaching in, their responses left him "devastated".
"Children spoke of being under pressure. One boy told me that when he was moved down a group, he couldn't speak to his mum 'for a couple of weeks'.
"Another boy spoke about feeling trapped - and it was across the spectrum.
"Before I carried out the research I would have assumed that children who were on the top-ability tables would be having the times of their lives and it was just us oblongs who didn't like it... but what I found was, whether children were on the top tables, the middle tables or the bottom tables, there was this universal feeling of pressure... related to that there was a fear of failure."
Some children even rejected extra support because they feared it showed they couldn't do the work and so would be moved down a group," says Mr Macnamara.
Disturbed by what he found, he tried to get senior management at the school to agree to let him trial mixed-ability teaching in his class - but the head vetoed the idea.
Dr Eleanore Hargreaves, who supervised his dissertation, herself an expert in the effects of ability grouping, says the vast majority of primary pupils in England are categorised according to their attainment in maths and writing and labelled by their schools accordingly.
In England, she says, streaming became widespread with the introduction of national assessment tests in the 1990s.
Supporters of ability grouping, such as John Blake, head of education at the Policy Exchange think tank, believe grouping children who need extra help "to provide such support" is perfectly sensible.
He says that, provided it is clear to teachers why the division has been made and the additional support required is given, there is no reason such grouping should be bad for children.
But Dr Hargreaves warns that when pupils are dubbed low ability, it's tempting for them to give up and decide that schooling and learning are not for them.
She argues that the concept of "innate" ability is highly disputable.
"'Talent' is often the result purely of intensive hard work and others' encouragement," she says.
"Certain children get more encouragement at school than others; and certain subjects get more emphasis.
"For example, teachers can tailor their teaching better to pupils from homes like their own and may not be able to relate so well to children from poorer families or those from different cultural backgrounds.
'Teaching to the top'
Last September, Mr Macnamara took a promotion as a deputy head at a school which does not use ability grouping.
Instead, he says, they "teach to the top" with all children in a class given the same tasks and those who struggle offered extra support to access the same work.
Ability grouping, where some children are given different or easier tasks risks them never understanding key parts of the curriculum and never reaching the required standard, he argues.
The school's latest Ofsted report backed this strategy but Mr Macnamara says he finds it frustrating that so many primary schools are still wedded to ability grouping.
"It is just so endemic. It's so entrenched that even to question it is heresy," he says.
"It's almost as if people don't know what to do other than a simplistic, 'Well, let's teach in ability groups.'"
Dr Hargreaves recently began a five-year study of pupils put in low ability groups aged seven. She will present details to UCL's Festival of Culture on Tuesday.