Recommender systems are increasingly supporting explanations to increase trust in their recommendations. However, studies on explaining recommendations typically target adults in low-risk e-commerce or media contexts, and using explanations in e-learning has received little research attention. To address these limits, we investigated how explanations affect adolescents' trust in an exercise recommender on a mathematical e-learning platform. In a randomized controlled experiment with 37 adolescents, we compared real explanations with placebo and no explanations. Our results show that explanations can significantly increase initial trust when measured as a multidimensional construct of competence, benevolence, integrity, intention to return, and perceived transparency. Yet, as not all adolescents in our study attached equal importance to explanations, it remains important to tailor them. To study the impact of tailored explanations, we advise researchers to include placebo baselines in their studies as they may give more insights into how much transparency people actually need, compared to no-explanation baselines.