For starters; your code is ripe for a vulnerability known as
SQL Injection, which was identified well over 20 years ago...
NEVER EVER
create an SQL command by string together together command text with user input- there is plenty of documentation on how to do add user content to a query
$stmt = $mysqli->prepare("SELECT username FROM attendance_student WHERE semester_class=?");
$stmt->bind_param("s", $_POST['name'], $semester_class);
$stmt->execute();
$result = $stmt->get_result();
References:
PHP: Prepared Statements - Manual[
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PHP MySQLi Prepared Statements Tutorial to Prevent SQL Injection[
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