How to pass id from template to controller in Ember?

I want to pass id from template user.hbs to controller user.js so I can bind student to a particular class.

It gives me an error: Assertion Failed: You cannot pass 'undefined' as id to the store's find method.

app/templates/user.hbs template:

<b> First name: </b> <br>
<b> Last name: </b>  <br>
<select>
        Liquid error: wrong number of arguments (1 for 0)
                <option value=>  </option>
        
</select>
<button > Insert student </button>

app/controllers/user.js:

import Ember from 'ember';

export default Ember.Controller.extend({  
  insertStudent(val){
                        var firstName = this.get('firstName');
                        var lastName = this.get('lastName');

                        var class2 = this.store.find('class', val);

                        var students = class2.get('students');


                        var newStudent = this.store.createRecord('student', {
                                firstName : firstName,
                                lastName: lastName,
                                class: class2
                        });

                        newStudent.save();

                        students.push(newStudent);

                        class2.set('students', studenti);
                        class2.save();

                        this.setProperties({
                                firstName: '',
                                lastName: ''
                        })
                }
 });