I have made a case study on the mathematical language status in an upper secondary school in Sweden. The participating pupils were asked to value some language-variables in mathematics and were given four assignments to solve. They were asked to describe their thoughts in writing in order for me to be able to determine what terminology they are using. The result shows that the pupils' mathematical language is insufficient and the prime ones in solving the assignments also are the ones using a more correct mathematical terminology, although not realising this fact themselves. Theories from linguistics and psychology that show the importance of developing the mind has a central role in this project, along with the works of the National Swedish Agency for Education to find explanations for Swedish pupils' ever decreasing figures in TIMSS and other international comparisons on students' mathematical proficiency.