Unable to insert data into database in dyanamic page

I'm creating a website which is for students in university to review their school. Since there are a lot of schools, I need to build a page that can take data from the database to show on the page, at the same time, I also put a form inside it, so they could comment and rate it. I identify each school by its school_id. This is what I'm trying:
This is the code the page before entering the dynamic page:

 <?php 
                $sql = "SELECT * FROM schools WHERE state_id=10001";
                $result = mysqli_query($con, $sql);
                if (mysqli_num_rows($result) > 0) {
                while ($row = mysqli_fetch_assoc($result)) {
                    echo"<form action='../school.php' method='post'><div class='states&school'>
                    <h3><button class='searchedschool' value=".$row['school_id']." name='id'>"
                    .$row['school_name']."</button>
                    </h3>
                    </div>
                    </form>";
                        }
                    }
                    ?>

And this is the code on the second page

 <?php             $sid=$_POST['id'];
                    if (isset($_POST['submit'])) {
                    $nickname=$_POST['user_nickname'];
                    $email=$_POST['user_email'];
                    $comment=$_POST['cmt_text'];
                    $course=$_POST['user_course'];
                    $rating=$_POST['user_rating'];
                    $school_id=$sid;
                    $db="INSERT INTO 
comments(user_nickname,user_email,cmt_text,user_course,user_rating,school_id) VALUES(?,?,?,?,?,?)";
                    $stmt=mysqli_stmt_init($con);
                    if(!mysqli_stmt_prepare($stmt,$db)){
                        echo "Data Error";
                    }else{
                        mysqli_stmt_bind_param($stmt,"ssssii", $nickname,$email,$comment,$course,$rating,$school_id);
                        mysqli_stmt_execute($stmt);
                    }
                    header("location:School.php");
                    exit;
                    }?>

This is the form

<form action="school.php" method="POST" id="myForm" class="form">
                <div class="row">
                    <div class="input-group">
                        <label for="name">Name</label>
                        <input class="typingbox"  minlength="5" type="text" name="user_nickname" id="name" placeholder="Enter your Name" required>
                    </div>
                    <div class="input-group">
                        <label for="email">Email</label>
                        <input class="typingbox" type="email" name="user_email" id="email" placeholder="Enter your Email" required>
                        </div>
                    <div class="input-group">
                        <label for="Course">Course</label>
                        <input class="typingbox" type="text"  minlength="6" name="user_course" id="course" placeholder="Enter your Course" required>
                    </div>
                    <div class="input-group">
                        <label for="rating">Rating (1-5) </label>
                        <input class="typingbox" type="number" min="0" max="5" name="user_rating" id="rating" required>
                        </div></div>
                    <div class="input-group textarea">
                        <label for="comment">Comment</label>
                        <textarea id="comment" minlength="50" class="typingbox" name="cmt_text" placeholder="Enter your Comment" required></textarea>
                    </div>
                    <div class="input-group">
                        <button name="submit" class="btn-btm">Submit</button>
                        <button type="button" class="btn-btm" onclick="closeForm()">Close</button>
                    </div></form>

This is one of the examples of how I present data from database

<h1 class="schoolheading"><?php
                    $result = mysqli_query($con,"SELECT school_name FROM schools WHERE school_id=$sid"); 
                    while($row = mysqli_fetch_array($result)) { 
                        echo $row['school_name'];}
                    ?></h1>

I didn't show the rest of the code because there's too many.

So basically every time I clicked the submit button, the data are not inserted into the database. Then I will be redirected to the same file but not showing any content and says that 'Undefined array key "id". How can I correct it? Or I should make my page the other way, as I'm not sure this is the correct way to make a dynamic page. I'm sure that my files are connected to the database.