In a more formal school, both the teacher and the pupil could greet the headmistress with "Good morning, Mrs X" or "Good afternoon, Mrs X". This is probably the safest choice for your assignment.
"Hello, Mrs X" would be slightly less formal, though probably fine in most schools.
"Hi, Mrs X" would be informal. In some schools, it may be acceptable, but the context you describe above is quite serious so I would go for something more formal.