I feel your pain. The same always happened to me at school and made me really anxious too. I think it's the combination of sudden anxiety, and also having to speak cold with no build up, and therefore no rhythm can be established. Sometimes I would talk quietly to myself beforehand to create a vocal rhythm in the hope that would work. Eventually what did work is I approached teachers beforehand to explain I had a stutter, and ask they mark me as present without me having to call out my stupid name. Hope this works for you. Let us know how you get on.