"if evolution is true AND God is true, why teach one but not the other?"
Excellent question.
I think we should teach both.
Science class is about what we can prove and use to create new technologies.
God should be taught to save souls.
Using God as an excuse to give up trying to understand the world he provided goes against the very notion of God giving us dominion over the earth - we are supposed to explore it and try ti understand it.
On a practical note, the ID in science class movement threatens our economy.
and since neiother evolution nor creationism can be proven, to teach one over the other censors the untaught, and endorses the unproven, eh?