In the introductory text, 'I go to Magura Govt. High School. It's an old school...' 'It' refers back to 'Magura Govt. High School', acting as a pronoun for a previously mentioned noun, which falls under describing a situation or characteristic related to it.
Class 6 › English Grammar and Composition › Chapter 7: Introductory 'There' & 'It' › Topic: Understanding the use of 'it' for time, day, month, date, weather, and situations
Aligned to the NCTB national curriculum.