Final answer:
The transition indicating 'mixed impact' is 'a) On the other hand,' as it contrasts differing points and shows a mix of positive and negative aspects.
Step-by-step explanation:
The transition that indicates the "mixed impact" of school uniforms in the section 'Mixed impact of school uniforms' is a) On the other hand. This phrase is commonly used to introduce a contrasting point or to discuss aspects that differ from previously mentioned information, thus showcasing a mix of both positive and negative elements. For instance, if one part of the text discusses the benefits of school uniforms, the phrase 'On the other hand' would introduce the drawbacks, thereby highlighting the mixed impacts of the topic under discussion.