public Teaches add(Teacher t, ClassGroup c) {
		Teaches newTeach = new Teaches(c, t);
		if (!this.teachingList.contains(newTeach)) {
			this.teachingList.add(newTeach);
			// If a object matching the new instance has been recently deleeted, remove that record.
			if (this.deletedTeaching.contains(newTeach))
				this.deletedTeaching.remove(newTeach);
			return newTeach;
		}
		return null;
	}