I ran a 1 day (10am - 4pm with lunch) Photoshop class at our local college but knocked it on the head after just two or three sessions. By the time you've done introductions your time is very limtied. I got the same feedback from both sessions I ran and the only way to fix it was to run a longer course.
1. Photoshop is huge. It's impossible to cover everything in a few hours so you end up skimming stuff by trying to cover everything and raising more questions than you can answer.
2. There are always one or two people who don't read the course info and simply cannot operate the computer. Do a course with windows PCs and everyone has a mac at home. And vice versa. This just sucks time up.
3. You get one person there to learn layers, and another person there to learn how to cut things out, and another person who wants to learn how to retouch old family negatives. Again, despite course info saying it's a very basic introduction, people expect that their specific needs will be met and are dissappointed when they're not.
I can see how short photoshop courses don't exist. I went on to run a 6 week (2 hrs/week evening class) course which was very popular but still suffered from the three issues above - just less so. I've since stopped teaching any courses on a computer. There are always people who expect you to include "operating a computer" in the couse too... A 16 week course would defintiely weed out those people and would be able to cover everything. However, as you say, much more expensive. It's likely to be the best face to face way though aside from private tuition.
My suggestion would be YouTube in bite sized sessions as and when you can. Layers? Cutting stuff out? Retouching? Compositing? Excellend videos online, and all can be done at your own pace.