I was in your same situation when I was in school. It was the anticipation and anxiety that made it even worse. For starters I would suggest letting your teacher know. Sometimes they are not always that attuned to it. Being called out by your teacher for now knowing a word is way worse than just trying to power through your stutter (I always hated class exercises like the one you described for that exact reason). Unfortunately I don't have much advice besides just telling you to power through it. Like other users have said on here, breathing exercises can help sometimes.