From Wikipedia: ( and I agree that underinvestment in non-grammars was at fault):
Design of the system
The basic assumption of the Tripartite system was that all students, regardless of background, should be entitled to an education appropriate to their needs and abilities. It was also assumed that students with different abilities were suited to different curricula. It was believed that an IQ test was a legitimate way of determining a child's suitability to a particular tier.
There were to be three categories of state-run secondary schools. Each was designed with a specific purpose in mind, aiming to impart a range of skills appropriate to the needs and future careers of their pupils.
Grammar schools were intended to teach a highly academic curriculum, teaching students to deal with abstract concepts. There was a strong focus on intellectual subjects, such as literature, classics and complex mathematics. In addition to wholly state-funded grammar schools, a number of schools currently receiving state grants could become direct grant grammar schools, with some pupils funded by the state and the rest paying fees.
Secondary technical schools were designed to train children adept in mechanical and scientific subjects. The focus of the schools was on providing scientists, engineers and technicians.
Secondary modern schools would train pupils in practical skills, aimed at equipping them for less skilled jobs and home management.
It was intended for all three branches of the system to have a parity of esteem. The appropriate type of school for each student would be determined by their performance in an examination taken in the final year of primary school.
The system in operation
Implementation
The Tripartite System was arguably the least politically controversial of the great post-war welfare reforms. It had been written by a Conservative, and had received the full backing of Prime Minister Winston Churchill.
Many in the Labour party, meanwhile, were enthusiastic about the ability of the Tripartite System to enable social mobility. A first rate education would now be available to any capable child, not simply a rich one. The tripartite system seemed an excellent tool with which to erode class barriers.
In spite of this broad approval, the resources for implementing the system were slow in coming. The logistical difficulties of building enough secondary schools for the entire country delayed the introduction of tripartite education. It was not until 1951, and the election of a Conservative government, that the system began to be widely implemented. Some historians[who?] have argued that tripartite education was the Conservative answer to the attractions of the Welfare state, replacing collective benefits with individual opportunities. Even so, there was still a dramatic shortfall in resources for the new education system.
Very few technical schools were opened, due to the lack of money and a shortage of suitably qualified teachers. This failure to develop the technical part of the system undermined the whole structure. The tripartite system was, in effect, a two-tier system with grammar schools for the academically gifted and secondary modern schools for the others.
Grammar schools received the lion's share of the money, reinforcing their
image as the best part of the system, and places in grammar schools were highly sought after. Around 25% of children went to a grammar school, although there was a severe regional imbalance, with many more grammar school places available in the South than in the North, and with fewer places available for girls. This was partly the result of a historical neglect of education in the north of England, which the tripartite system did much to correct. Nevertheless, in 1963 there were grammar school places for 33% of the children in Wales and only 22% of children in the Eastern region.
Modern schools were correspondingly neglected, giving them the appearance of being 'sink schools'. Although explicitly not presented as such, the secondary modern was widely perceived as the bottom tier of the tripartite system. They suffered from underinvestment and poor reputations, in spite of educating around 70% of the UK's school children.
The Newsom Report of 1963, looking at the education of average and below average children, found that secondary moderns in slum areas of London left fifteen year olds sitting on primary school furniture and faced teachers changing as often as once a term.
Existing beliefs about education and the failure to develop the technical schools led to the grammar schools being perceived as superior to the alternatives. The system failed to take into account the public perception of the different tiers. Whilst officially no tier was seen as better than the other, it was a generally held belief amongst the general public that the grammar schools were the best schools available, and entry into the other two types was considered a "failure".
Alongside this system existed a number of public schools and other fee-paying educational establishments. These organised their own intakes, and were not tied to the curricula of any of the above schools. In practice, most of these were educationally similar to grammar schools but with a full ability range amongst their pupils.
