To execute environmental education effectively, the success and impacts of educational activities must be assessed. In areas of high biodiversity there is generally a lack of evaluation of the impact of environmental education. In this study we investigate the effect of a one-time classroom activity on student knowledge of local environmental issues, environmental attitudes and aspirations. The project was conducted on the island of Maio, Cabo Verde, a small, highly biodiverse island, as part of a classroom visit programme with the local environmental organization, Maio Biodiversity Foundation. We visited every fourth-grade class (i.e. students with an age range of 9–10 years) on the island (a total of 142 students) and delivered a half-day classroom activity. The results show that this activity did not influence environmental attitudes; however, it did significantly improve student knowledge and awareness of local environmental issues. This study shows that environmental education should not be assumed to improve attitudes and knowledge but requires individual evaluation for each type of activity. For environmental education to reach its full potential, activities should be planned in response to evaluation results.