This article aims to evaluate and analyse the degree of efficiency in secondary school teaching based on a survey carried out in 36 classes of ''seconde'' maths (fifth year). The first result concerns the ''teacher effect'' : when comparing the performance of students on common exercises (between groups of equal age, level of education, sex and social background) there was found to be a large discrepancy depending on the class and the teacher (the marks varied from 5 to 20 in the more extreme examples). This difference is analysed as the result of the ''expectation effect'', in other words, the sort of mark that will be awarded for the exercise. There are two main results: The marks awarded to the pupil are not always based on their real performance but according to the teacher's marking habits. more or less generous. When all factors (initial level, age, sex, social background) are equal the more generous marking habits imply better performance in end-of-year testing. These results underline the need for a more global reflection on the effects of context and the role interactions play in the development of success or failure at school.