ALTER TABLE student_fees
ADD CONSTRAINT fk_teacher_staff
FOREIGN KEY (teacher_id) REFERENCES staff(staff_id) ON DELETE SET NULL;

ALTER TABLE student_fees 
ADD teacher_id INT NULL,
ADD class_id INT NULL;

ALTER TABLE students ADD COLUMN teacher_id INT NULL AFTER student_class;

ALTER TABLE students 
ADD monthly_fee DECIMAL(10,2) NOT NULL DEFAULT 0;