CLOSE to 9,000 children are today due to take the first of this year's 11-plus exams.
The Association for Quality Education (AQE) said it received an "unprecedentedly high" number of entries.
There have been about 8,700 registrations for its Common Entrance Assessment (CEA).
Grammar schools remain split into two camps, using either the CEA or multiple-choice papers set by GL Assessment.
As many children take both exams, they must sit separate papers on four consecutive weekends.
There has been no state involvement in 11-plus exams in a decade.
A total of 34 - all non-Catholic - grammar schools use the CEA papers to fill their places.
AQE said it had to run three supplementary centres this year to meet the demand for spaces, as result of the higher birth rate.
Schools, the group said, had a limit to the number of candidates they could accommodate.
This means some children must attend Belfast Met's Titanic campus or South Eastern Regional College in Lisburn instead of their preferred school today.
On Saturday November 17, thousands of children will take the GL Assessment papers. They are used by 26 Catholic grammars, six non-Catholic grammars and two integrated schools.
Five of the non-Catholic schools that accept GL results, also award places to those with CEA scores.
The heads of Catholic primary and secondary schools object to the continued use of transfer tests.
The Catholic Principals' Association has said it is opposed to "the unregulated system of social selection at 10/ 11 years of age which operates under the guise of academic testing".
It said no part of Catholic education should be involved in supporting or operating a system of selection which promotes social segregation and inequality.
The second and third CEA papers will be taken on November 24 and December 1.
It had been hoped that a new single assessment would be in place by autumn 2018.
A discussion paper published in March 2017 suggested children should face three 45-minute test papers. Two papers would be multiple choice while the third would instead contain writing and maths exercises. All three could be taken on a single day.
The Irish News this year revealed that almost £30,000 of public money was spent trying to establish a single 11-plus - even though the Department of Education had "no role". Fees and expenses were paid to three experts asked to shake-up the unofficial transfer test system.
