Mathematics is central to the STEM subjects (science, technology/computer science, engineering, and mathematics) and also to a wide range of non-STEM subjects. Accordingly, it is important that first-year students have adequate mathematical knowledge when they enter higher education. However, it remains unclear whether high school students are aware of the relevance of mathematics in higher education and what, if any, prior mathematical knowledge is relevant in different subjects. Although high school students were found to consider mathematics to be highly relevant, they limited this perceived benefit only to basic mathematics. In addition, university students were found to have incorrect or inadequate ideas about the mathematical content required for and covered in university study programs, which can lead to unfulfilled expectations and, in the worst case, to dropping out of university. This study therefore aims to complement the university students’ perspective with the school students’ perspective. To this end, we investigated 984 high school students’ conceptions of (1) the relevance of mathematics for future studies at a university in general and (2) the specific mathematical requirements in particular subjects. The results show that most students underestimate the relevance of mathematics in several subjects. In detail, we found students to have various vague and inadequate ideas about the specific mathematical requirements of particular subjects. In social sciences, and to some extent in economics, the mathematical requirements were largely underestimated. Such misconceptions may lead students to choose mathematics-intensive subjects when they actually want to avoid mathematics in their university studies.