Do not have students shout out answers in voice when you work as a whole class in larger classes. It is very difficult to hear who is saying what even if you know the voices of your students. The quality of voice also differs greatly depending on SL that day, on bandwidth, on the qualitiy of the participants microphone, etc. 

Good alternatives:

1. Have students write in local chat (eg. T or a student asks in a game: Which is the longest river of the world?) All students type in their answers in to local chat as fast as they can. This way everybody has the same chance and it is easy to see who was first.

2. If you have “hand show chairs“, which are available in-world for free”, use these. Students who sit on these chairs can raise their hand by simply clicking on one key on their keyboards and everybody can see who raises their hand. 

I do prefer the first option, though, as it is easier and can be done everywhere without having to set up or rez any objects.