If i am a teacher with a CCW and "a good guy with a gun" small enough to conceal

[Recursion]

17. A few things

Assault weapons aren't in particular "large" (in fact, they're smaller than traditional rifles), and the size of a weapon doesn't really say anything about its power.

If you were to buy a handgun today, it would probably be a semi-automatic with a detachable magazine, which means it would have the exact same rate of fire as the assault weapon. In fact, it was normal, everyday handguns that Cho used at Virginia Tech, and these are also the weapons used in the majority of murders and the vast majority of murders by firearm.

why is there a need to manufacture the assault weapons

Because people like owning low-power light-weight rifles with external designs that are less than a half-century old.