Some people think of working memory as that kind of short term memory you use when your just trying to keep information in mind for a short time...like remembering a phone number that somebody tells you before you write it down. But the work in working memory implies that you're going to put the information to some other purpose, whether it's applying it to another task, analyzing it, or changing it.
In the figure below, look at the developmental differences that occur when working memory is really put to work (e.g. digits repeated back in reverse order vs. simply repeated).
Needless to say, that's why 8-12 year olds can become as swamped as they can doing working memory-demanding tasks like listening to and acting on instructions (listen, comprehend, see, do) or doing multi-stepped mathematical problem solving (see, remember, retrieve math facts and procedural steps, write). At older grades, critical reading and writing, note-taking, scientific and mathematical problem solving, and foreign language dialogue may be a student's Waterloo. Look at the increase in errors and slowed reaction time that occur with increased demands on working memory.
Many assume that a limited working memory would limit academic success, but many an absent-minded professor (e.g. Nobel Prize Winning physicists Enrico Fermi, Norbert Wiener) seemed to have this trait. These folks made up for this weakness by their tenacity of intellectual pursuit, reliance on note or human reminders, and strong long term memory stores.
In the classroom, working memory weaknesses can be circumvented by practice with strategic note-taking or previewing material, computer-based learning (text and visual illustrations reinforce each other), and other self-paced learning. Elaborative learning is also reinforcing, but for younger kids, auditory working memory demands may mean elaboration should be presented in separate chunks (over time), in a spiraling fashion, or with text-picture reinforcement.
Developmental Changes in Working Memory
Reading Skills for University: Critical Thinking
Elaborative Questioning, Knowledge, and Interest, Learning from Texts
A Picture's Worth 84 Words