Should children as young as three, four and five be taught by ability?
Grouping children like this in nursery school, Reception and Years 1 and 2 is increasingly common across England, according to a report by University College London's Institute of Education and the National Education Union.
The study raises concerns about the impact of teaching by ability on pupils' confidence and aspirations.
But while some are critical of the practice, others say it works well.
So what are the different views on the issues of grouping children in the early years, Reception and Key Stage 1?
In the Grouping in Early Years and KS1 "A necessary evil"? report, which was based on responses from 1,400 union members, authors Dr Guy Roberts-Holmes and Dr Alice Bradbury claim the practice of grouping young children has become "taken for granted".
The report says school leadership teams expect teachers to group pupils and the practice is seen as preparation for tests such as the phonics screening check and KS1 Sats tests.
"Many teachers think children are aware of their group, and they are concerned about the detrimental impact of being labelled on children's self-confidence and behaviour, and about the production of limits on children's learning," it says.
"There were concerns raised that this could have a potentially negative impact upon some children's mental health.
"Teachers both in early years and KS1 feel that the pressure of assessments produces the need for grouping in some form."
The research quotes a teacher from a focus group who recalled a girl telling her that her aspiration to become a doctor, like her mother, had disappeared when she was moved down a set.
Some people feel that grouping children so the work is targeted to their individual needs makes perfect sense.
John Blake, head of education at the think tank Policy Exchange, says: "If children need additional support to understand the key concepts of learning, then grouping them together to provide such support is perfectly sensible.
"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 - quite the reverse, in fact, since schools should be using whatever tools they think are appropriate to ensure children have learnt the curriculum.
"Parents should be worried if schools were avoiding teaching children appropriately, using effective methods, including ability grouping if the school judges that to be appropriate."
Mr Blake says it is a teacher's job to make sure that children don't feel inadequate or stressed.
"There is no reason at all that teachers should be passing on to children any stress or pressure about the phonics screening check or their Sats tests - these are not 'high stakes tests' for the students.
"Indeed, many schools never even tell the children they have been through a statutory test at all. I find it extraordinary that teachers think that children are stressed and it's not the pupils' fault."
Mother-of-two Elisa felt let down when she found out her son had been grouped by ability - into a lower group - from Reception.
"It was like he'd been branded - 'you belong to the lower group and you'll never move up to the higher group'," she said.
"I couldn't understand why some of the mums were so pushy with their children in Reception and Year 1 - it was because they wanted to make sure they were in the advanced group.
"Children develop at different times, so to say at five or six that's what you are, you're not as smart as everyone else and you belong to a certain group and you're stuck with it is just not fair - you're condemning those children to say at that stage rather than pushing them on."
Elisa says the experience made her son doubt himself.
"He called himself not as intelligent as the other children, he wasn't as clever as so-and-so. He found it hard with his self-esteem, I think."
Elisa says she's now much stricter with her second child, who is a summer-born.
"I'm much less relaxed with her - she has to work a lot harder, I'm not going to let her fall behind. I tutor her, I'm not going to take any risks whatsoever - my trust with school after my experience with my son was lost."
The head teacher
Some head teachers say the practice of grouping children helps them as professionals to target help, especially as some children start school "ready to go" and others need a lot of nurturing.
One school leader quoted in the UCL/NEU report said: "I personally think it's better for the children, because otherwise your more able children get bored and frustrated and your less able children just get left behind.
"So the grouping means that you can focus your attention."
Whether and how teachers arrange any grouping of pupils is very much down to individual schools. Some schools might move children between groups fairly loosely, others might be more regimented.
Minister for children and families Robert Goodwill said: "Teachers and early years staff are best placed to make decisions about the teaching methods they use.
"There is no statutory requirement that suggests children should be grouped by ability."
Teaching primary-age children by ability can be done in a number of ways, the UCL/NEU report says.
Streaming: Children are put in a class based on a view of their ability.
Setting: Children are placed in groups for particular subjects, usually literacy and maths, and move from their normal mixed-ability class for this subject.
Within-class ability grouping: Ability groups are used within a class - usually sitting at different tables with different tasks and levels of support. This may occur in a mixed-ability class, or within a set.
Interventions: Specific children are targeted and removed from the class for additional support or extension activities. This is often for a fixed period of time and a specific purpose, for example booster groups.