I teach several classes for a photography school.
The first thing to know, is 'what is the scope of the class'? For example, is this a class that anyone can take (with any type of camera), simply to learn to take better photos? Or will this be a class only for those who have an SLR (or only a DSLR)? If you want to be teaching how to shoot in manual mode, you'll likely have to exclude those who don't have an SLR camera.
But, if you are going to teach a class for those who have a DSLR, there are plenty of other things that will probably have to be taught before/with manual exposure etc. For example, how to focus, focus modes, MF vs AF, focus points etc. Teaching them about exposure is probably the most important thing you can teach them, but for the full effect, they need to know about the three exposure settings and their consequences....so ISO/noise, shutter speed/motion-blur-sharpness and aperture/DOF.
All that may take up to 10 hours of class time. Keep in mind that you'll have to be somewhat familiar with different camera brands & models, so that you can help students when they ask camera specific questions.
So just as an example, our school has several classes. One is a very basic, 'How to take better photos'. Another is 'Using your DSLR'. Another is 'Creative Vision and Photographic Design'....and there are many more.
So my point is, you likely can't (shouldn't) just have a 'photography' class. You probably need to be more specific as to the scope/intent of the class and go with that.
Well, you can do it however you want I guess. You could just have a quick class that goes over the basics and then take them on a guided photo walk....and just answer their questions as they come up.
As for the number of students, that depends on what & how you're teaching. We like to keep most of our classes to 12-16 students, allowing for plenty of time for questions and some one-on-one attention. But we have some classes that we limit to 8 students, because of the increased amount of personal attention needed, and the space/props that are used etc.