For several years, the Miami-Dade County Public schools had toyed with replacing some of its 1,000 diesel buses with cleaner electric vehicles. But school leaders said the change would be too costly. Then 12-year-old student Holly Thorpe showed up at a school board meeting to tout the benefits of going electric and returned to encourage the district to apply for a state grant. She explains how she’s been dismayed to see the environment around her deteriorating: “I’ve just always like loved the planet and loved animals and like water. So like seeing trash around and like seeing how it’s like dying just always broke my heart. So I’ve just always been interested in like trying to help the planet. Two years on, the school board is considering a district plan to use state money to replace 50 diesel buses with electric models over the next several years. Holly’s stand has …