The error lies in the first part of the sentence. After superlative adjectives, majorly two prepositions are used: of and in. We use in with a singular word referring to a place or group. Eg: I am the happiest woman in the world. We use of before plurals, before time periods such as year/month. Eg: Sunday is the best day of the week. Here, in this case, the subject is the 'school', thus, it should be 'in the school'.
