You must be calm. Shouting and screaming, though justified, shows that you cannot get your point across without raising your voice.
Be calm, clear but very very firm. The headmistress must know that this has happened more than once. What you dont want to do is not have her on your side, and defending the teachers. Plus you have to think that your children will be in this school, so you dont want the teachers to act funny with your children because you have made a complain about them
Though my child is not at school yet, trust me, I would be extremely annoyed and I would be in the headmistresses office straight away.
Let us know how you get on!!!!!