public Enrolment addEnrolment(Student s, ClassGroup c) {
		Enrolment enrol = new Enrolment(s, c);
		if (!this.enrolments.contains(enrol)) {
			this.enrolments.add(enrol);
			// If a matching enrolment has recently been deleted, remove that record of the object.
			if (this.deletedEnrolments.contains(enrol))
				this.deletedEnrolments.remove(enrol);
			return enrol;
		}
		return null;
	}