$row = mysql_fetch_assoc($query);
$dbactive = $row['active'];

if ($dbactive == 1){

...

}...

// don't use $mysqli->prepare here
        $query = "SELECT TeacherForename, TeacherSurname, TeacherUsername, TeacherPassword, Active FROM Teacher WHERE TeacherUsername = ? AND TeacherPassword = ? LIMIT 1";
        // prepare query
        $stmt=$mysqli->prepare($query);
        // You only need to call bind_param once
        $stmt->bind_param("ss",$teacherusername,$teacherpassword);
        // execute query
        $stmt->execute();
        // get result and assign variables (prefix with db)
        $stmt->bind_result($dbTeacherForename,$dbTeacherSurname,$dbTeacherUsername,$dbTeacherPassword, $dbActive);

            while($stmt->fetch()) {

      if ($teacherusername == $dbTeacherUsername && $teacherpassword == $dbTeacherPassword) {
   if ($dbActive == 1){

        $loggedIn = true;
}
      }

}

$dbactive = $stmt['active'];

$result = $myslqi->query($query);
while($row = $result->fetch_object()){
  $row->active …
}

if($query = $db->prepare("your sql query where field1=? AND field2=?")){

    $query->bind_param('ss', $user_id);
    $query->execute();
    $query->bind_result($field1, $field2);
    $query->fetch();
    echo $field1;

}