Class Management

case 'create_class':
    $class_name = trim($_POST['class_name']);
    $stmt = $pdo->prepare("INSERT INTO classes (teacher_id, name) VALUES (?, ?)");
    $stmt->execute([$teacher_id, $class_name]);
    $_SESSION['success'] = "Class created successfully";
    break;

function addStudent($pdo, $class_id, $student_id, $teacher_id) {
    // Verify class belongs to teacher
    $stmt = $pdo->prepare("SELECT id FROM classes WHERE id = ? AND teacher_id = ?");
    $stmt->execute([$class_id, $teacher_id]);
    if ($stmt->rowCount() === 0) {
        $_SESSION['error'] = "Invalid class selected";
        return;
    }
    
    // Check if student exists
    $stmt = $pdo->prepare("SELECT * FROM students WHERE student_id = ?");
    $stmt->execute([$student_id]);
    if ($stmt->rowCount() > 0) {
        // Check if student is already in one of THIS teacher's classes
        $stmt = $pdo->prepare("
            SELECT cs.class_id, c.name 
            FROM class_students cs
            JOIN classes c ON cs.class_id = c.id
            WHERE cs.student_id = ? AND c.teacher_id = ?
        ");
        $stmt->execute([$student_id, $teacher_id]);
        $existing_enrollment = $stmt->fetch();
        
        if ($existing_enrollment) {
            if ($existing_enrollment['class_id'] == $class_id) {
                $_SESSION['error'] = "Student is already enrolled in this class";
            } else {
                $_SESSION['error'] = "Student is already enrolled in your class: " . $existing_enrollment['name'] . 
                                    ". A student can only be enrolled in one class per teacher.";
            }
        } else {
            $stmt = $pdo->prepare("INSERT INTO class_students (class_id, student_id) VALUES (?, ?)");
            $stmt->execute([$class_id, $student_id]);
            $_SESSION['success'] = "Student added successfully";
        }
    } else {
        $_SESSION['error'] = "Student ID not found";
    }
}

case 'remove_student':
    $stmt = $pdo->prepare("DELETE FROM class_students WHERE class_id = ? AND student_id = ?");
    $stmt->execute([$_POST['class_id'], $_POST['student_id']]);
    $_SESSION['success'] = "Student removed successfully";
    break;

try {
    if ($file_extension == 'csv') {
        $reader = new \PhpOffice\PhpSpreadsheet\Reader\Csv();
    } elseif ($file_extension == 'xlsx') {
        $reader = new \PhpOffice\PhpSpreadsheet\Reader\Xlsx();
    } else {
        $reader = new \PhpOffice\PhpSpreadsheet\Reader\Xls();
    }
    
    $spreadsheet = $reader->load($_FILES['excel_file']['tmp_name']);
    $worksheet = $spreadsheet->getActiveSheet();
    $rows = $worksheet->toArray();
    
    // Skip header row if it exists
    $start_row = (isset($_POST['has_header']) && $_POST['has_header'] == 1) ? 1 : 0;
    
    // Begin transaction
    $pdo->beginTransaction();
    
    $imported = 0;
    $skipped = 0;
    $errors = [];
    
    for ($i = $start_row; $i < count($rows); $i++) {
        $row = $rows[$i];
        
        // Skip empty rows
        if (empty($row[0])) continue;
        
        $student_id = trim($row[0]);
        
        // Check if student exists
        $stmt = $pdo->prepare("SELECT * FROM students WHERE student_id = ?");
        $stmt->execute([$student_id]);
        $student = $stmt->fetch();
        
        if (!$student) {
            // Create new student if we have enough data
            if (!empty($row[1]) && !empty($row[2])) {
                $name = trim($row[1]);
                $email = trim($row[2]);
                $default_password = password_hash('changeme123', PASSWORD_DEFAULT);
                
                $stmt = $pdo->prepare("INSERT INTO students (student_id, name, email, password) VALUES (?, ?, ?, ?)");
                if (!$stmt->execute([$student_id, $name, $email, $default_password])) {
                    $errors[] = "Failed to create student: $student_id";
                    continue;
                }
            }
        }
        // ... enrollment logic
    }
}

$message = "Successfully imported $imported students.";
if ($skipped > 0) {
    $message .= " Skipped $skipped students.";
}

$_SESSION['success'] = $message;

if (!empty($errors)) {
    $_SESSION['import_errors'] = $errors;
}

// Get students in the class
$stmt = $pdo->prepare("
    SELECT s.student_id, s.name, s.email 
    FROM students s
    JOIN class_students cs ON s.student_id = cs.student_id
    WHERE cs.class_id = ?
    ORDER BY s.name
");
$stmt->execute([$class_id]);
$students = $stmt->fetchAll();

// Create spreadsheet
$spreadsheet = new \PhpOffice\PhpSpreadsheet\Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();

// Add headers
$sheet->setCellValue('A1', 'Student ID');
$sheet->setCellValue('B1', 'Name');
$sheet->setCellValue('C1', 'Email');

// Add data
$row = 2;
foreach ($students as $student) {
    $sheet->setCellValue('A' . $row, $student['student_id']);
    $sheet->setCellValue('B' . $row, $student['name']);
    $sheet->setCellValue('C' . $row, $student['email']);
    $row++;
}

function deleteClass($pdo, $class_id, $teacher_id) {
    // Begin transaction to ensure all operations succeed or fail together
    $pdo->beginTransaction();
    
    try {
        // Verify ownership
        $stmt = $pdo->prepare("SELECT * FROM classes WHERE id = ? AND teacher_id = ?");
        $stmt->execute([$class_id, $teacher_id]);
        if ($stmt->rowCount() > 0) {
            // Delete attendance records for this class
            $stmt = $pdo->prepare("DELETE FROM attendance WHERE class_id = ?");
            $stmt->execute([$class_id]);
            
            // Delete schedules for this class
            $stmt = $pdo->prepare("DELETE FROM class_schedules WHERE class_id = ?");
            $stmt->execute([$class_id]);
            
            // Delete students from class
            $stmt = $pdo->prepare("DELETE FROM class_students WHERE class_id = ?");
            $stmt->execute([$class_id]);
            
            // Delete class
            $stmt = $pdo->prepare("DELETE FROM classes WHERE id = ?");
            $stmt->execute([$class_id]);
            
            $pdo->commit();
            $_SESSION['success'] = "Class deleted successfully";
        } else {
            $pdo->rollBack();
            $_SESSION['error'] = "You don't have permission to delete this class";
        }
    } catch (Exception $e) {
        $pdo->rollBack();
        $_SESSION['error'] = "Error deleting class: " . $e->getMessage();
    }
}

<div class="info-banner">
    <strong>Note:</strong> Students can be enrolled in multiple classes from different teachers, but only in one class per teacher.
</div>