The reading for 大 is the おお reading (like 大きい). Nothing you can do about that but remember it. Then, 勢 is rendaku'd to become ぜい. Also something you'll just have to remember. Say this word a few times in your head, then you'll be able to come up with an idea of what sounds right and what sounds wrong so that you can figure it out next time this word comes up.
I’m not used to speaking in front of crowds, so my hands are already shaking.
When I take the attendance of a large number of students, I sometimes skip over one or two names.