I would ask your teacher to show you the exact MO state statue he is referring to. He won't be able to. This is an argument that has been used other places, but it is still wrong. A CCW permit allows a person to conceal upon their persons a weapon. This is because it specifically states in the MO state statues that it is illegal to do so otherwise. There is no where in the statues that say if you chose to obtain a permit you are forever kept from carrying in any other manner. It's not a one or the other kind of thing. Here is a link to the MO Bill Of Rights, Article 1, Section 23
And a link to MO State Statues Chapter 571. It is the chapter on Weapons Offenses. No where in there shall you find that is is illegal to carry openly if you have a CCW permit.
Hope this helps