Anti-LGBTI+ harassment in schools is a worldwide problem. Several OECD countries support civil society organisations working directly with students to raise awareness about LGBTI+ inclusion in classroom sessions lasting a few hours. However, a rigorous impact assessment has never been conducted on any of these interventions so it is not known if they contribute to reducing anti-LGBTI+ harassment - and therefore whether they should be scaled up, given that they are currently only available in a handful of volunteer secondary schools. This document presents the results of a groundbreaking randomised control trial conducted in the Paris region of France from 2018 to 2022 with over 10 000 students aged 13-18 to measure the impact of sessions by SOS homophobie, the main French association in the fight against anti-LGBTI+ discrimination and violence. The results reveal it is possible to sustainably improve students' receptiveness to LGBTI+ inclusion during two hours of structured but totally open discussion. In doing so, our conclusions contribute to refute two common misconceptions: (i) no, these sessions are not too short to have any impact; and (ii) no, they are not counterproductive just because homophobic and transphobic opinions can be voiced. While it is true that negative group dynamics can emerge, they are neither systematic nor powerful enough to overshadow the positive effect of SOS homophobie's intervention.