Using the value of dropdown

[trololol]

Good day sir.
I've been able to populate the dropdown using the value from database but i don't know how to use them to query and display some information.

Here's the code:

 
<form method="GET" action="try.php">
School :
<select name="school" class="schools">
<option selected="selected">--Select School--</option>
<?php
include('connection.php');
$sql=mysqli_query($db,"SELECT school FROM subjects GROUP BY school");
while($row=mysqli_fetch_array($sql))
{
$school=$row['school'];
echo '<option value="'.$school.'">'.$school.'</option>';
} ?>
</select>
&nbsp;
 
Course :
<select name="course" class="courses">
<option selected="selected">--Select Course--</option>
<?php
include('connection.php');
$sql=mysqli_query($db,"SELECT course FROM subjects GROUP BY course");
while($row=mysqli_fetch_array($sql))
{
$course=$row['course'];
echo '<option value="'.$course.'">'.$course.'</option>';
} ?>
</select>
 
</form>