When I teach, I often need to ensure that specific knowledge is understood by everyone in the class..
There is a few free online systems at the internet to help me with that. This one is PollEverywhere.
If you in the registration, remember to choose that you are a teacher in the education sector, then 40 people can respond to each survey you create. (I have only occasionally had a need for more people to answer, and then I have chosen to use another program called IQpoll - it can do up to 100 people for free).
It is completely anonymous.