How can i post a list object to API by using Checkbox and Angularjs?

My problem is:
I have a list Student in a table with checkbox. Now i want to check this box and send a list Student to API. But i cannot to get them.
My code here!

HTML:

<tr dir-paginate="student in ClassStudents | filter: search | itemsPerPage: 20">
    <td><input type="checkbox" ng-model="student.selected"></td>
    <td style="display: none;">{{student.Class.ClassId}}</td>
    <td>[Another property]</td>
</tr>

AngularJS:

function selectStudents(){
    var listStudent = [];
    $scope.ClassStudents.forEach(function(student){
    if(student.checked){
        var studentClone = angular.copy(student);
        listStudent.push(studentClone)
        }
    })
    return listStudent;
}
$scope.upGrade = function(){
    $http({
        method: 'POST',
        url: host + "/api/APIClassStudents/Post",
        data: {
            Students: selectStudents(),
            ClassId: 2
        }
    }).then(function(response){
        alert("Upgrade Successful!")
    })
}

Connect to WEB API is ok, response Upgrade success! but nothing change. When i debugging, i think selectStudent()[now is a array] is count = 0. And i dont know why.
Thanks for help!

Source: AngularJS