David - There may also be a more dangerous effect at play. Those who don't attend THINK they know more than they do while those that are attending embody the "the more I know, the more I know I don't know" principle. In other words: those that don't attend have ego's that exceed their expertise: In my opinion, this is the most dangerous human condition n software.
In a conversation a few years back with Steve McConnell after conducting a code-complete class Steve said something like that the best developers rate the Code Complete class class highly while those that rate the class poorly are almost always developers who under perform.
One suggestion is to extend the group learning idea is book study. Meet once a week, agree a book or sections of a book to be studied. Each week agree the chapters that will be prepared before the next meeting. Then discuss each week for an hour with a discussion leader. As a manager (just as a professor might do) you can place pressure on all the individuals to answer directed questions which will expose how well prepared they are. Note that as a manager - who can affect compensation - injecting yourself int the training will affect behavior - just as the professor - who assigns grades - does. Excusing yourself will generate more true learning behavior from the team - the result being that those wanting to learn will learn more and those less interested in learning will learn less. You as a manager need to choose what is better - cater to your weakest links or strengthen your strongest?
It may also be important to align the study topics with general objectives. As an example, if the organization makes an annual objective of strengthening the use of , say UML, in development then individual objectives can be tied to this. The study classes should be focused around UML subject areas. This creates a linkage that should incent most people to attend - if not they are likely to miss their objectives.
I would bet that those not attending will prove to be under performers that will need the right level of management action. The sooner that happens, the better.
/andrew