@api.model
def get_students_in_a_class(self):
    _logger = logging.getLogger(__name__)
    # _logger.info('Class id'  + repr(val.x_class_id) )
    parents = self.env['parents'].search([('partner_id','=', self.env.user.partner_id.id)])
    _logger.info('Parents ' + repr(parents))

    sessions = []
    classes = []
    students = []

    for parent in parents:
         students = self.env['op.student'].search([('parents_ids','=', parent.id)])
         _logger.info('Students ' + repr(students))

         for student in students:
            classes = self.env['x_classes'].search([('id','=', student.x_class_id.id)])
            _logger.info('Classes ' + repr(classes))

            for clas in classes:
                sessions = self.env['sessions'].search([('class_id','=',clas.id)])
                _logger.info('Sessions ' + repr(sessions))

    d = dict()
    d['Classes'] = classes
    d['Sessions'] = sessions
    d['Students'] = students
    _logger.info('Data-->> ' + repr(d))
    return d

odoo.define('javascript_module.demo', function(require){
 "use strict"

 var model = require('web.Model');
 var studentModel = new  model('op.student');

 var data = studentModel.call('get_students_in_a_class')
 console.log('data--->>>>>>>', JSON.stringify(data));

})

Server application error
{"message":"Odoo Server Error","code":200,"data":{"debug":"Traceback (most r
recent call last):n  File "E:\Oddo\Odoo 10.0\server\odoo\http.py",
line 640, in _handle_exceptionn  File "E:\Oddo\Odoo
10.0\server\odoo\http.py", line 691, in dispatchn  File
"E:\Oddo\Odoo 10.0\server\odoo\http.py", line 629, in
_json_responsen  File "json\__init__.pyc", line 244, in dumpsn  File
"json\encoder.pyc", line 207, in encoden  File "json\encoder.pyc",
line 270, in iterencoden  File "json\encoder.pyc", line 184, in
defaultnTypeError: op.student(113, 115) is not JSON
serializablen","exception_type":"internal_error","message":"op.student(113,
115) is not JSON serializable","name":"exceptions.TypeError","arguments":
["op.student(113, 115) is not JSON serializable"]}}