Upper-secondary school students must prepare for adult life, which--among others--entails acquiring relevant skills and discovering their own potential. Efforts at European and national levels have been made to ensure that students gain the competences, the so-called key competences, which facilitate functioning in the modern world. However, in Polish upper-secondary schools their overall development may be problematic as schoolwork is heavily exam-oriented. In order to address this challenge, a study was undertaken, the aim of which was to investigate the usefulness and feasibility of applying gamification to an extracurricular CLIL project intended to develop key competences in an upper-secondary school. The paper reports on one part of this study, i.e. on how a biology challenge (two tasks) was designed, implemented and evaluated. The results show educational and emotional gains, suggesting the motivational effect of gamification in learning.