I run a cubing club at my elementary school. Most of the students who come are 4th-6th grade. Each year I will typically have 30-40 students learn to solve a cube, though I've had it up to 80 one year. Keep in mind that this is elementary, but the biggest thing that made the club popular was having the Rubik's Cube Certificate and keeping a bulletin board in a high traffic area of the hallway. If a student had demonstrated that they could solve the cube without help or guides, I would take their picture and fill out a certificate for them, which would go on the bulletin board in the hallway. The title of the bulletin board was "You Can Do the Cube". I would also put other pictures of different types of cubes or cubing logos and pictures of students in the club. Students in a middle school might not want this kind of exposure, but in elementary cubing became the popular thing. Another big thing was whenever a student asked me anything about cubing I would tell them about the club and ask them to come and to bring a friend.
The way I ran the club it was more of a 3 day class and then you could come back if you wanted or not. Those who came back would help others who were beginning. Some of the veterans would time themselves on cubetimer or, if we didn't have any beginners, I would teach them to solve another puzzle.
If you really want people to get into it, it is much better to have a small supply of 2x2's, 3x3's, 4x4's and maybe Pyraminxes. Most 6th graders won't go to great lengths to try and convince their parents to buy cubes from China, but if you have it there at the school, they will bring wads of cash. For 2x2 I sold LanLans for $6, 3x3 Ghosthands for $8, 4x4 mini-QJ for $11 and QJ Pyraminxes for $8. For much of the year it was hard just to keep them in stock. Many people will buy them who don't have any interest in learning how to solve it.
I also have access to a projector and a big screen, so I show the students youtube videos of world record solves or just fun cubing videos.
I'd say the biggest thing I learned about successfully teaching people to solve a cube is having some kind of a guide to go with it, so if they get stuck at home they can use the guide. I took the pictures from the You Can Do the Cube site and made my own booklet. Their booklet was very wordy and students don't like to read a whole lot, so my instructions were very simple and basic. If new people come, the first thing I would do is let them know there is no easy trick to solving it, it will take between 3-4 1-hour sessions, though some people have done it in 2 or even 1 and others have taken up to 5. They need to commit to solving it and not give up easily. When teaching it, the slower you go the better. If some students are quicker, they can use the guide at home and get ahead. The first day teaching the cross was always the hardest part and pretty much took most, if not all, of the first session. Of course, the first 20-30 minutes is just teaching them about the different types of pieces and how a cube is solved in layers, not by sides. Then spending time teaching the students the notation with pictures and have them practice just following the pictures. I'd run through all the moves twice and make sure everyone knew them. Then when teaching the cross, I'd make sure they could solve the cross 3 times completely on their own before moving on. This goes well with any of the steps.
The way I run the class though is with the goal that everyone can learn to solve the Rubik's Cube, not just elite, gifted people and trying to get everyone possible to come to the class.
Anyway, let me know if there are any other questions I could answer.
. I kid, but seriously this is cool people are interested in starting clubs
