I'd recommend you to start without any framework like flixel, flashpunk or even Box2d. They do really make life simpler to you by keeping in mind the ancient programmer philosophy "Don't always reinvent the wheel"
If you start programming games with just flash, these would be your plus points
- You learn little bit about the essential computer graphics necessary algorithms, ideas and programming logic.
- You make a lot of mistakes and learn a lot more when correcting them.
- You do think of "making" the game logic, rather than allowing the framework to do it for you.
If you start with a framework, these are the plus points
- They provide you with a plethora of built in functions which help a lot in the game logic.
- They make life simpler.
- You would get more time to invest in building your core game logic.
But, If you start programming games in flash with a framework, these are minus points
- You wouldn't need/try to understand the algorithm behind the game logic implementation.
- You may/would find it difficult when changing to another language for a similiar game programming need if that particular language doesn't have similiar library features as the one you use.
To sum it all up, If I were a teacher I'd ask my students to start learning Game Programming without a framework.Using an SDK is recommended though.Flex SDK is useful if you were thinking of programming in FlashDevelop IDE or other Flash IDEs.