English Translation
Mr. Smith always starts his lessons by making a joke.
Making a joke usually means mocking something, trivializing a subject that is actually serious. People tend to find it offensive. Maybe M. Smith is mocking physics (or whatever he teaches). There are some subjects that it could probably work well!
I did find in Larousse: faire une blague à quelqu’un = to play a joke on somebody (© Larousse 2014).
I did a learn a bit from delving into this. If it is supposed to be “telling a joke“, maybe raconter would be a better verb? As always, I’m not sensitive so feel free to disagree or correct me. I want the right information out there.