Teacherly advice about humor

Encouraged to look in my Facebook "other" folder, I found a message from someone I didn't know, but it's great advice from a teacher who taught a class before me, I wish I'd gotten something similar when I first started teaching:

"Be tough (and funny) with my students also avoid that 20 percent of them gave up their first lesson about scuba diving. Cruelty is an action intended to cause pain, to injure others. Being tough (always with humor in my case) when necessary and for the sake of learning is profitable if relevant. I think"

I agree but don't care for the "avoid" statement, assuming I follow the meaning. I try to push/challenge such students, but it's balancing act. Usually you mention something, and let it go until they come to you.  Otherwise, students become resentful, no matter how cheerfully you say it.

Teachers need to be trained in humor and hecklers. Just my opinion, not legislation.  But serious un-sidetrack-able humor. That's the danger the students want to exploit. 

I now open my classes with jokes to get them in a good mood. Who knows what aggravations they're carrying from home or another class?  The common wisdom is to start class immediately, but maybe a good mood greases the wheels better.