We are professionals and if we act like them, we will get paid like them. Rather than cattle herding the Bucket List divers, if you focus on smaller numbers of students, keeping them in the industry, discounting for continuing loyal customers as they move up the classes, you will make better profit.
Turn them out as great divers and keep them happy and excited for diving. They will come back then.
They won't be the diver trampling the reef and bragging that diving is easy and perpetuating that idea.
They will even recommend you! They will buy gear from you, even if it's priced slightly higher.
As g1138 hinted at, the best "tip" I have received thus far is getting to see some of the divers I trained continue to dive. Every time one of those divers comes back to take a continuing education class or ask me about the gear he wants to buy, it tells me that I must have done a good job. When one of those divers become so excited about diving that he wants to get his entire family certified, putting them in my next class is one of the highest compliments I could receive on his class experience. After one of those divers goes on the "best dive trip ever", getting to see his pictures and listen to his stories from the trip is going to make me smile months or years after his class ended.
At the end of the day, I'm usually not going to remember the fact that a student left me a few dollars though.
That being said, I don't necessarily think that tipping (monetarily) during classes is a bad thing if you (the student) feels that it is warranted. Do I believe that tips be expected or asked for? Definitely not.
Many of the instructors I learned from would use any and all tips they received to take care of their DMs/assistants, regardless of whether they these people had been tipped separately. This could take the form of buying lunch, coffee, or drinks for everyone after the class or putting a tank of gas in the DM's car a week later. Knowing how much a nice lunch or a much needed cup of coffee can brighten the day of someone who has been working their a** off, I see nothing wrong with accepting a tip so that I can better take care of the people who have helped me. Teaching a class is very rarely a solo effort/achievement and sometimes a small tip is a great way of recognizing that fact. On the other hand, I have also had the experience of working with instructors who actively asked for or suggested tips. I have not, and will not, work with them again.
To the OP, there is no cut and dry rule about tipping your instructors/DMs so do what you feel is right in your situation! *Since it was a rescue class, if you were responsible for the breakage of any body parts that are not your own, you may want to at least buy that person a drink though...