For those interested in 3D (though, it can be used in 2D) game making with an engine, I would suggest teaching Unity. It's an engine and a platform for creating games.
You seem to be focused on game development, which is probably the right thing to do since at least some of your student colleagues will also be interested. But there are other possibilities to keep in mind, such as some simple information system that your school might use. Teachers might be able to give you ideas about that.
Since you want the club to ...
There are commercial flavours of the BASIC language that specialise in creating games; I remember briefly playing with Blitz Basic many many years ago and found it easy to work with, but there are numerous competing products you might want to also consider. Despite dramatically reducing the complexity of creating games, absolute beginners are still going to ...
I've had a go doing this. The activities I tried were:
Taking screenshots of yourself wearing the skin and
showing in a web page.
Making custom blocks.
Taking screenshots of a
building that uses the new block and showing as a web page.
or houses. ok, it's not lua :)
Python (probably Python 3) would be a great option. Python is a very user-friendly language, perfect for beginner programmers (it also enforces good practices like indentation, etc). That being said, Python has many fantastic tools for experienced programmers, and is a very flexible language. It also has the already mentioned Pygame extension. Python is also ...
Show them the cool stuff they can do with Scratch.
Don't focus on explaining that they can't make a AAA game. Focus on showing them examples of stuff that is engaging and interesting, that they can do with Scratch. Come up with a showcase of interesting programs. Make sure your assignments are engaging and interesting.
Taking a step back, I'd also ...
Here is a link to a magazine helloworld written for teachers of computing (k→13). It is free to download as pdf, and free on paper to teachers in the UK. It has some good articles on the use of play; copying and modifying; working in groups; and many other ideas.
Also observe how your teachers teach. What do they do well. What could they do better. Do not ...
Let them start by playing games that are actually teaching them programming concepts and are mostly not AAA games. None of this uses Scratch, but it is fun and it is learning programming which is also problem solving.
Human Resource Machine
or problem solving,
I would probably say Python or Ruby, being that they are fairly easy languages to read and learn for beginners while still being worth their salt for CS pros.
We used processing in our summer program. It's a 4 week full day Mon-Fri program for rising 9 - 12 graders with no experience.
In that time frame, they're able to learn some good CS fundamentals and write a nice project and games are the most common projects.
Processing isn't a game development platform per se so that is both an advantage (teaching core cs ...