STEM (Science, Technology, Engineering, Mathematics) subjects and careers have been historically triggering rather low interest among high school students. This goes completely against their growing relevance for society and increasing demand by the public and private sectors. Various approaches have been proposed to promote STEM subjects to high school audiences. Hands-on experiments were shown to be successful for student learning in science and for creating a positive attitude towards STEM, which can also influence future career choices. Here, we propose a citizen science association where high school students will act as school scientists in water resources research. Citizen science has become increasingly popular in research as it offers the potential for substantial, thoughtful public outreach, combined with research problems that scientists themselves could not tackle as they are too laborious or costly. Within the proposed SCALE project around 300 school scientist from high schools around Luxembourg will carry out – in collaboration with scientists – an extensive precipitation sampling campaign to understanding the spatio-temporal variability of stable isotopic composition in rainfall at small scales. This will expose the students to experimental design, carrying out the experiment, and scientific data analysis. Besides having clear scientific benefits, the proposed project will promote the scientific method and shed light on professions in science to students with various backgrounds. This should contribute to paving the way towards new approaches in Geography teaching at secondary schools in Luxembourg and for promoting STEM careers among Luxembourgish high school students. We expect the project to also trigger long-term impacts – ultimately leading students that may embrace STEM careers to become the next generations of experts with interdisciplinary skillsets, urgently needed for tackling pressing environmental issues.