const TeachSchema = mongoose.Schema({
  name: { type: String, required: true },
  email: { type: String, required: true },
  username: { type: String, required: true },
  password: { type: String, required: true },
  created: {type: Date, default: Date.now},
  classes: [
    {
      cName: String,
      student: [
        {
          name: { type: String, required: true },
          lname: { type: String, required: true },
          gender: { type: String, enum: ['M', 'F'] },
          grades: { type: Array }
        }
      ]
    }]
});

{
        "_id" : ObjectId("5c0c55642440311ff0353846"),
        "name" : "Test",
        "email" : "test@gmail.com",
        "username" : "test",
        "password" : "$2a$10$RftzGtgM.DqIiaSvH4LqOO6RnLgQfLY3nk7UIAH4OAvvxo0ZMSaHu",
        "created" : ISODate("2018-12-08T23:36:04.464Z"),
        "classes" : [
                {
                        "_id" : ObjectId("5c0c556e2440311ff0353847"),
                        "cName" : "1A",
                        "student" : [
                                {
                                        "grades" : [
                                                1,
                                                2,
                                                5,
                                                7
                                        ],
                                        "_id" : ObjectId("5c0c55812440311ff0353848"),
                                        "name" : "StudentName",
                                        "lname" : "StudenteLastName",
                                        "gender" : "M"
                                }
                        ]
                }
        ],
        "__v" : 0
}

db.teachers.update({"_id": ObjectId("5c0c55642440311ff0353846"), "classes._id": ObjectId("5c0c556e2440311ff0353847"), "classes.student._id": ObjectId("5c0c55812440311ff0353848")},{$addToSet: {"classes.$.student.grades":6}})

Teacher.findByIdAndUpdate(
    {_id: req.user.id,
    "classes": {$elemMatch: {_id: classe}},
    "classes.student": {$elemMatch: {_id: req.params.id}}
    },
    {$addToSet: {"classes.$.student.0.grades": voto}},
    (err, result) => {
        if (err) {console.log(err)}
        else {console.log(result); res.json(result)}
    }
)