For young people, schools are platforms for learning and strengthening competencies about nature preservation; despite being faced with the same learning opportunities, learning outcomes are very heterogeneous. We were interested in environmental knowledge gains and retention after participation in a student-centred learning programme, and in environmental attitudes' role in knowledge acquisition. 261 students participated in a 90-minute programme about the forest ecosystem and related sustainability topics. Our 8-station in-classroom module relied on a collaborative, hands-on, and autonomy-supportive learning approach. We measured knowledge three times: before the programme (pre-test), right after programme completion (post-test), and six weeks after programme completion to test long-term performance (retention test), using the Rasch model. Knowledge scores increased in the post-test and decreased in the retention test but remained significantly above pre-test scores. Environmental attitude had a considerable effect on knowledge scores of each test time, though effects on pre- and retention scores were larger. This indicates that students with positive attitudes knew more before the programme and retained more knowledge over time. Interestingly, the retention scores of those with the most positive attitudes exceeded their post-programme scores, which suggests further immersion into the topic, rendering attitude a probable and promising lever for long-term knowledge acquisition.