Whats the most beneficial way a class can be taught to be efficient as possible, and so the people being taught retain as much as possible?

Depends on many factors.
- what is the training requirement and what are you training?
- existing subject knowledge in the training audience
- is this the first time they are seeing the task?
- is this subject conceptual or is it hand on? If it is hands on, it may be more effective to pre-test and use those people as assistants. Demonstrate and explain the task as you go, then show it at full speed. Then have the Training audience members test out on the task themselves.
- can you exercise the task/subject in the conditions it will be executed?
- do you have all the training resources? Facilities, location, equipment, required supplies, additional instructors?
- the training audience receptiveness has the most influence of long term retention. If they find it important or are especially in to it, then they retain it. If they are training hostages with no buy in, they'll do what they have to to "get through it", then forget it.
- one time training is not effective. Repeating the task being trained helps with proficiency and retention. A technique is to include the trained task in related task training as a building block or to put it in context. Example: if you are training operation of a 256 kit in training session one, training session two may be quartering party operations, so you run the individual task doing the collective task or session 2 is unmasking procedures with and without 256.

People's interest, participation, and learning retention only lasts further than 30 seconds in if they see relevance. That's the connecting the dots piece to doing better, getting the job done easier, making life in the badlands more tolerable. The list goes on. Where the Services fail greatly is in not keeping the relevance piece centered on the endless EEO, SHARP, GMT, etc. stuff that isn't related to pulling triggers better. What's in it for me if I go along with this stuff? Do I really need to be on board with this stuff to get promoted? When the Services lose touch in demonstrating the results of the sand in the gears by not paying attention to larger picture issues, you no longer see how your life is better by pushing forward on these topics as well. Probably the most paid attention to schools I went to were Blasting and SERE (Survival Evasion Resistance Escape). The immediate nature of cause and effect was always up front. As the topics get softer, then you have to up the relevance game more.

Hands on can be a big help in retention of material covered, if it works. Class needs to be engaged in the process of learning.