HTML <select> not submitted - php

Question

I'm trying to submit a form to my PHP validation script and I get 2 undefined indexes. Both of which are an select tag (school and function, focus on the function for this question). I've looked through several posts, and I still don't see what's wrong.
Here's the form:
<form action="actions/user_edit.php?id='. $user['id'] .'" method="post" id="form_editUser" class="col s12">
<div class="row">
<div class="input-field col s12">
<input value="'.$user['name'].'" id="name" name="name" type="text" class="validate">
<label for="name">Naam</label>
</div>
<div class="input-field col s12">
<input value="'.$user['email'].'" id="email" name="email" type="text" class="validate">
<label for="email">Email</label>
</div>
<div class="input-field col s12">
<select id="school" name="school">
<option>Ignore this select kthxbai
<option>
</select>
<label>School</label>
</div>
<div class="input-field col s12">
<select id="function" name="function">
<option value="" disabled>Kies functie</option>
<option value="Admin" >Admin</option>
<option value="Directeur" >Directeur</option>
<option value="Docent" >Docent</option>
</select>
<label>Functie</label>
</div>
</div>
</form>
<div class="modal-footer">
<input type="submit" value="Bewerk" form="form_editUser" class="modal-action modal-close waves-effect waves-green btn">
Annuleer
</div>
Here's the PHP script:
require '../config/db.php';
$userId = $_GET['id'];
$name = mysqli_real_escape_string($conn, $_POST["name"]);
$email = mysqli_real_escape_string($conn, $_POST["email"]);
$schoolId = mysqli_real_escape_string($conn, $_POST["school"]);
$function = mysqli_real_escape_string($conn, $_POST["function"]);

Answer 1

Turns out my form was inside a tbody element, which is not allowed. Problem solved, thanks for all the help!

Answer 2

Make sure to check your $_POST[] variables for existence, eg.
$option = isset($_POST['school']) ? $_POST['school'] : false;
if ($option) {
$schoolId = mysqli_real_escape_string($conn, $_POST["school"]);
}
I have seen cases where if nothing was selected as an option, the same is not included in the POST payload itself.