Validate registration of a teacher in a course and a PHP section

I have a question that is giving me some headaches. I'm doing a system where I register courses (violin, guitar, flute ...) and each course will have a section (section A, section B, section C ...). Each course will have a day and a schedule of related shifts according to the section (if it is section A it will be only Monday and Wednesday from 8am to 12pm .. if it is section B it will be Tuesday and Thursday from 3pm to 5pm, and so ..) .

The problem comes when registering a teacher in a section and a course, once I register the teacher, I can select it by combobox after filling in the information of the section with their hours And days, my question is How can I prevent a teacher who is already in a course from 8am to 12pm only on Mondays and Wednesdays can not be registered again for example in a section C where they are on Tuesdays and Wednesdays at the the same time that the section A? I want to avoid is the clash of hours per teacher when registering it in a section and course.

A teacher can give many courses, but not in the same shift (morning-evening) on the same day. But if you can take a course in the morning shift and another in the afternoon shift.

For now I have my BD this way:

CREATE TABLE 'seccion' (
  'id_seccion' int(11) NOT NULL,
  'descripcion' varchar(10) DEFAULT NULL,
  'horaEntrada' varchar(10) DEFAULT NULL,
  'horaSalida' varchar(10) DEFAULT NULL,
  'dias' varchar(50) DEFAULT NULL,
  'cupos' int(10) DEFAULT NULL,
  'curso_id_curso' int(11) DEFAULT NULL,
  'personas_cedula' varchar(13) DEFAULT NULL,
  'turno' varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

and try to do the following, but as I said, it gives me headaches and how to place this that I consult:

$result1 = $mysqli->query("SELECT COUNT(personas_cedula) FROM seccion WHERE personas_cedula='$cbx_profesor' AND turno='matutino' AND dias='$dias' AND horaEntrada='$horaEntrada'  ");
                    $row = @mysqli_fetch_row($result1);
                    if($row[0]>=1){


                        $errors[] = "El profesor ya se encuentra registrado en un curso";

                    } else {





                    $sql = "INSERT INTO seccion (descripcion, horaEntrada, horaSalida, dias,cupos,curso_id_curso,personas_cedula,turno) VALUES('$descripcion','$horaEntrada','$horaSalida','$dias','$cupos','$cbx_curso','$cbx_profesor','$turno')";
    $resultado = $mysqli->query($sql);

to anyone who can help me, I would greatly appreciate it in advance!