Adding a validation check to a form in a while loop php

I have a form which displays like this: Event Name : Drop down menu

I am trying to add a check that ensures that each event produced by the while loop has a student assigned to it - by selecting from the drop down menu.

I have attempted adding a check for this but its not making a difference - it loads form action page 'savecompetitors'.

I have got this for php so far:

<?php
    session_start();
    require_once 'db/connect.php';
    require_once 'db/checkuserloggedin.php';
    include 'db/header.php';    

    echo $_SESSION['Username'] . ' logged in successfully';
    echo '<h3> Entry form </h3>';

        //Query to display all events                           
        if ($event_result = $con->query("SELECT Name FROM event")) {    

            echo "<form method =\"POST\" action=\"savecompetitors.php\">";
            echo '<table>';
                while ($row = $event_result->fetch_assoc()) {
                    echo '<tr>';                        
                        echo '<td>';                                                    
                        echo $row['Name']. ' ';                                 
                        echo '</td>';

                        if ($student_result = $con->query("SELECT Student.Form, Teacher.Form, Forename, Surname, Student_ID " .
                                                            "FROM student, teacher " .
                                                                "WHERE Student.Form = Teacher.Form AND Teacher.Username = '" . $_SESSION['Username'] . "'")) {

                            if ($student_result->num_rows) {                                                                                                                                        
                                echo '<td>';                                                                                            
                                    echo "<select name ='". $row['Name'] ."'>"; 

                                    while ($row1 = $student_result->fetch_assoc()) {                                        
                                        echo '<option value="" style="display:none;"></option>';
                                        echo "<option value ='" . $row1['Student_ID'] . "'>" . $row1['Forename'] . ' ' . $row1['Surname'] . "</option>";

                                    if (isset($_POST['submit'])) { 
                                        if (empty($_POST['Student_ID'])) {
                                            $error = 'A student must be selected for every event';
                                        }
                                    }

                                    }

                                    echo "</select>";
                                echo '</td>';
                                echo '</tr>';   
                            }
                        }   
                }           
            echo '</table>';    

            echo '<input type="submit" name="submit" value ="Submit" >';
            echo '<input type="reset" value ="Reset">';
            echo '<span class="error"><?php echo $error;?></span>'; 
            echo '<span class="error"><?php echo $success;?></span>';   

            echo "</form>";     
        }

        else {
            echo 'No student records available';
        }   
?>

savecompetitors php:

<?php
    require_once 'db/connect.php';
    $error='';
    $success='';

            $event_result = $con->query("SELECT Event_ID, Name from event");
            while ($row = $event_result->fetch_assoc()){

                $eventname = str_replace(' ', '_', $row['Name']);
                print_r($row);

                $con->query("INSERT INTO competitors (Event_ID, Student_ID) VALUES (" . $row['Event_ID'] .", " . $_POST[$eventname] .")   ");
                $success='Entry form has been successfully saved and students are entered as competitors for their submitted events';           
            }       
?>