Demo Code: Search in JSON - The full code for StackOverflow question #74668608 :)

schools.dart

import 'dart:convert';

/// Core School class. Details about individual School.
class School {
  String city;
  String districty;
  String name;
  School({required this.city, required this.name, required this.districty});

  factory School.fromJson(Map<String, dynamic> json) {
    return School(
      city: json['city'],
      name: json['name'],
      districty: json['district'],
    );
  }
}

/// Schools class to organize search and additional functionalities
class Schools {
  List<School> list;
  Schools({required this.list});

  factory Schools.fromJson(List<dynamic> json) {
    return Schools(
      list: json.map((e) => School.fromJson(e as Map<String, dynamic>)).toList(),
    );
  }

  static Future<Schools> get instance async {
    final String response = await rootBundle.loadString('assets/Schools.json');
    List data = await json.decode(response);
    return Schools.fromJson(data);
  }

  List<School> searchByCity(String city) {
    return list.where((element) => element.city == city).toList();
  }
}

// Use it like:

void main() async {
  Schools schools = await Schools.instance;
  List<School> schoolsInIstanbul = schools.searchByCity('ISTANBUL');
  
  print("We have ${schoolsInIstanbul.length} schools in Istanbul");
}