---------------------arquivo aluno.json - pasta Json
{
    "id":"1245",
    "nome":"Paula Soares",
    "nota" : 90
}

--------------------arquivo aluno.dart - pasta model
class Aluno {
  
  String alunoId;
  String alunoNome;
  int alunoNotas;

  Aluno({
    this.alunoId,
    this.alunoNome,
    this.alunoNotas
  });

   factory Aluno.fromJson(Map<String, dynamic> parsedJson){
    return Aluno(
      alunoId: parsedJson['id'],
      alunoNome : parsedJson['nome'],
      alunoNotas : parsedJson ['nota']
    );
  }

}

------------- arquivo aluno_service.dart - pasta services

import 'dart:convert';
import 'dart:async' show Future;
import 'package:flutter/services.dart' show rootBundle;
import '../model/aluno.dart';

Future<String> _carregaAlunoJson() async {
  return await rootBundle.loadString('Json/aluno.json');
}

Future carregaAluno() async {
  String jsonString = await _carregaAlunoJson();
  final jsonResponse = json.decode(jsonString);
  Aluno aluno = new Aluno.fromJson(jsonResponse);
  print(aluno.alunoNotas);
}

--------------------arquivo main.dart - pasta lib

import 'package:flutter/material.dart';
import '../services/aluno_service.dart';

void main() {
  runApp(new MyApp());
  carregaAluno();
}

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        home: Text("Teste")
    );
  }
}

------------------arquivo pubspec.yaml

 assets:
     - json/aluno.json