Tripartite System

The Tripartite System of education in the United Kingdom was a system of education introduced in the Butler 1944 Education Act, covering Primary and Secondary education.

The system was based on two premises:

That education should be fair, with the brightest given the full range of opportunities regardless of their background.
That the intelligence of a person was Academic, Technical or Practical.

Based on these premises, the Tripartite System worked as follows:

All pupils would attend Primary school up to age 11.
At age 11 a test called the eleven plus would be taken, which was a specialised IQ test would determine which type of intelligence each child had.
Based on which type of intelligence a child had, they were sent to a grammar school (Academic), Secondary Technical school (Technical) or Secondary Modern school (Practical).

It was hoped that this would mean that each child got the type of education most suited to them, and that bright working class children would get a decent education where previously they would have been denied one.

However, the system suffered from the following disadvantages:

Those pupils classed as Technical weren't catered for very well, as not many Secondary Technical schools were built. As a result Technical children went to Secondary Modern schools in most cases.
The eleven plus institutionally discriminated against girls. It was official policy to mark down female scores, so there were girls on the borderline of the Academic threshold who were denied a Grammar school education just because of their sex.
The aforementioned lack of Secondary Technical schools meant that it became a two-tier system of Grammar schools and Secondary Modern schools.
There was supposed to be parity of esteem between the three types of school, with none being better than the other two, just different. However, the fact that there were only two types of school in practice and the fact that the marking down of girls resulted in them going to Secondary Moderns meant that it became a pass/fail system, with Grammar being the pass and Secondary Modern the fail.
70% of children went to Secondary Modern schools and 5% to Secondary Technical schools. Coupled with the pass/fail argument above, this meant that 75% of children failed, and the Self-Fulfilling Prophecy of being labelled as failures (however indirectly) meant they did not do as well.
The middle class got the most benefit out of the system, to the expense of the working class. The more friendly stance of middle class subculture to education (both in interacting with educational institutions and in making sure their children were better educated, both through their own efforts and getting other pople to help) meant that middle class children had an advantage on the eleven plus and as such there were far more middle class children in Grammar schools than working class.
The value of IQ tests has been questioned. Inparticular the eleven plus was seen as culturally biased towards middle class people (for example it might ask a question which related to classical composers, something a middle class child would be more likely to answer right than a working class child), but all IQ tests have some bias towards one culture or way of thinking.