$schedule=Array
(
    [2017-12-19] => Array
        (
            [week_day] => Вторник
            [directions] => Array
                (
                    [Вальс] => Array
                        (
                            [Группа 2] => Array
                                (
                                    [19:00:00-20:00:00] => 19:00:00-20:00:00
                                    [18:00:00-19:00:00] => 18:00:00-19:00:00
                                )

                            [Группа 1] => Array
                                (
                                    [20:00:00-21:00:00] => 20:00:00-21:00:00
                                )

                        )

                    [Танго] => Array
                        (
                            [Группа 2] => Array
                                (
                                    [18:00:00-19:00:00] => 18:00:00-19:00:00
                                )

                        )

                )

        )

{"2017-12-19":{"week_day":"Вторник","directions":{"Танго":{"Группа 2":{"19:00:00-20:00:00":"19:00:00-20:00:00"}},"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2017-12-21":{"week_day":"Четверг","directions":{"Танго":{"Группа 2":{"19:00:00-20:00:00":"19:00:00-20:00:00"}},"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2017-12-20":{"week_day":"Среда","directions":{"Вальс":{"Группа 1":{"19:00:00-19:00:00":"19:00:00-19:00:00"}}}},"2017-12-26":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2017-12-28":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-02":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-04":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-09":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-11":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-16":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-18":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-23":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-25":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-01-30":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-01":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-06":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-08":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-13":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-15":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-20":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-22":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-02-27":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-03-01":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-03-06":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-03-08":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-03-13":{"week_day":"Вторник","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}},"2018-03-15":{"week_day":"Четверг","directions":{"Вальс":{"Группа 1":{"20:00:00-21:00:00":"20:00:00-21:00:00"},"Группа 2":{"18:00:00-19:00:00":"18:00:00-19:00:00"}}}}}

foreach ($schedule as $date) {
    ksort($date['directions']);
}