How to save form input array in mongodb?

This is the form I am using :

<form method="post" enctype="multipart/form-data">
  <label class="label">Class Name</label>
  <label class="input">
    <input type="text" name="class_name[]">
  </label>
  <label class="label">Grade</label>
  <label class="input">
    <input type="text" name="grade[]">
  </label>
</form>

Below is the schema I am using :

var SchoolSchema = new mongoose.Schema({
  classroom: [{
    Name: {
      type: String,
      required: true
    },
    grade: {
      type: Number,
      default: 1
    }
  }]
});

Firstly I tried saving the data using below function :

var School = new mongoose.Schema.model('School');

exports.submit = function(req, res) {
  var classRooms = [];
  for (var i = 0; i < req.body.class_name.length; i++) {
    classRooms.push({Name: req.body.class_name[i], grade: req.body.grade[i]});
  }
  School.create({classroom: classRooms}, function(err, school) {
    ...
  });
};

It worked fine when there was only a single entry in the form But for multiple entries it enters all the entries in single document. For e.g if through form I enter

Name: ABC Grade:1 Name: EFG Grade:2

This data dets saved in data base as

classroom:[{Name:ABC,EFG, Grade:1,2}]

and it should be like

classroom:[{Name:ABC, Grade:1},{Name: EFG, Grade:2}]

I even try to change the function as below:

var School = new mongoose.Schema.model('School');

exports.submit = function(req, res) {
  var classRooms = [];
  for (var i = 0; i < req.body.class_name[0].length; i++) {
    classRooms.push({Name: req.body.class_name[0][i], grade: req.body.grade[0][i]});
  }
  School.create({classroom: classRooms}, function(err, school) {
    ...
  });
};

Now this worked fine for multiple entries, but when I enter a single entrie it didn't worked as expected. For e.g. If I enter following data through form,

Name: AB Grade: 2

It gets saved in the database as

classroom:[{Name:A,Grade:2 },{Name:B,Grade:1}]

Would anyone please let me know what I am doing wrong here and how to correct this. Thanks !!