Unclear admission rules mean many summer-born children in England are missing a year of education or starting school too early, campaigners claim.
The group, Flexible School Admissions for Summer Born Children, says the school starting age for summer babies has in effect become four, not five.
Under current law, children in England must be in education from the term after their fifth birthday.
But the law also allows for pupils to start school earlier.
As a result, the vast majority of children begin their education by taking up a Reception class place at the age of four.
A Department for Education spokeswoman said that admissions rules had been changed to make it easier for parents to defer entry and that schools and councils must make this clear in their admissions arrangements.
In a report, the Flexible School Admissions for Summer Born Children group says evidence suggests that children born in the summer months are not always ready for school and are more likely to face social, emotional and academic challenges.
The report claims a lack of clarity in the school admissions code means that it has been misinterpreted by many schools and councils, which are not sticking to the legislation behind it.
Consequently, schools and councils have developed their own policies and practices, which have made it difficult for children to start school at any time other than joining Reception at age four or Year 1 the September after their fifth birthday.
"Essentially, in the process of affording parents the choice of enrolling their four-year-old children in school prior to compulsory school age, the primary education legislation that still says parents can wait until the term after their child turns five has effectively been forsaken," the report says.
The group says it is calling on ministers to make it easier for parents to choose whether to send their summer-born child to start school at age four or in the following September after they turn five.
It should also be simpler for parents sending their child to school at age five to choose whether they join a Reception class (often seen as the final year of early years education) or Year 1 (usually seen as the first year of formal schooling).
The report claims parents who wait and send their summer baby to school at age five are usually forced to enrol their child in Year 1, missing out on the Reception year, unless they can prove that there are exceptional circumstances for their child to join the Reception classes.
The group says this should not be the case and that parents should have a choice between the two school years.
The report also says that some parents currently feel pressured to apply for a school place for when their child is four, fearing they will run the risk of not getting place at all in Year 1, if all the places have been allocated to children in Reception.
Report author Pauline Hull said there should be less focus on a child's chronological age and more emphasis on their developmental age.
Ms Hull said: "We want flexibility for all summer-borns. The ones that are ready for school can start early and those that are not then start at compulsory school age - age five.
"It is irrefutable that summer-born children can lawfully start school at age five in reception class and experience a full primary education, but by failing in its responsibility to ensure that all primary legislation and EU law is fully communicated and adhered to by every admission authority, the government is allowing admissions discrimination and unfairness to continue."
A DfE spokeswoman said: "We have changed the School Admissions Code so that it is more flexible for parents of summer-born children, making it easier for them to defer their child's entry.
"Parents should also have the flexibility for their children to attend part time until they reach their fifth birthday or request their child enters reception class rather than Year 1, following their fifth birthday.
"Schools and councils must make this clear in their own admissions arrangements - and we have recently published guidance to re-iterate these responsibilities.
"We are working closely with school admission authorities to make sure they are acting within the rules and we will not hesitate to intervene where this is not the case."