Or even worse, trying to pick up materials at the end of class while students are asking all sorts of questions about when assignments are due, what they missed, etc. With practice it has become easier to attend to these multiple tasks but I would say that it is still far from automatic or easy. Each situation is a little different than the one before. A good example of where divided attention improved with practice is skiing. In the beginning, I found it extremely difficult to attend to bending my knees properly, holding my poles just so, watching the approaching terrain, etc.
Selective attention involves attending to one message or stimulus and ignoring all others.
Yesterday, at the party I was at, there many different conversations going on. I had to focus on the conversation I was involved in, selectively listening to the person talking to me rather than all of the other conversations going on around us. Sometimes, when I am trying to selectively attend to one thing and ignore another, I am unsuccessful. For example, yesterday, as I was driving home from the party, I of course was trying to attend to my driving and the other cars on the road and suddenly noticed I was worrying about some problem instead of attending to my driving. First, the basic principle of practice and attention seems to be that with practice the attentional resource requirements of a task diminish.