Dunraven School in Lambeth held the test for 300 children from local primary schools last month although its request to introduce selection for half its intake had been turned down by Education Secretary Gillian Shephard.
The school's head, Richard Townsend, insists that the test results will be used, not to help the school select the brightest children, but as evidence to support a fresh application to select pupils.
He said: "The decision from the Education Secretary came through very late, but we went ahead with our test because parents were expecting it. The results will be used internally to give an indication of each pupil's abilities. They will not be used to allocate places. In future years, the tests may be held after the places have been offered, but it will be up to the governors to reconsider."
Dunraven wants to select half its intake on general ability with the rest drawn from the local area. It plans to resubmit its bid, claiming the support of the Funding Agency for Schools, HMI and parents.
The three-part test, which consists of non-verbal spatial awareness questions to assess IQ, a written numerical test and a verbal question-and-answer session, is the same as the one taken by entrants to nearby Graveney School in Wandsworth, which won its selection bid in December.
Graveney, Burntwood School (another Wandsworth school which was approved for selection at the same time) and Dunraven had hoped to set up a common entrance test for all three schools.
Dunraven wants the change in order to achieve a balanced intake of abilities and help stem the flow of Lambeth pupils to other boroughs. Mr Townsend said the test results had already thrown up interesting factors by highlighting a wide gap between pupils' IQ and their verbal and numerical abilities.
The head says the tests reveal a worrying lack of achievement by children when they transfer to secondary school. He refuses to blame local primary schools but will be meeting with them to discuss the findings, which he hopes will add weight to the resubmission.
A spokeswoman for the DFE said: "We did not know the school was carrying on with the tests. This area is fraught with legal difficulties and we are looking into it.
"The school was refused permission to select because the Education Secretary was not convinced it was educationally justified and she believed it could affect parental choice in the area."
The spokeswoman said there was no political reason for accepting two schools in Wandsworth and refusing one in Lambeth - each school was judged on its merit. But parents had complained to the DFE that making Dunraven partly selective might change its character. The school had committed itself to staying comprehensive when it opted out in April 1993.
In last year's examination performance tables, 21 per cent of Dunraven pupils scored five or more higher grade GCSEs (A*-C). This compares with a Lambeth average of 22 per cent and an English average of 43.3 per cent.
Last week The TES published details of budget reserves held by more than 800 GM schools in England. Dunraven held the biggest surplus, with Pounds 926, 650 in the bank at the end of March 1994.