If we don't do this, then just as we have adults who are deficient in writing and science, we will find that adults won't have the character and the values needed to be decent members of the community or decent employees or decent soldiers. Most people feel that this question is such a strong counter-argument that after they ask it, they can walk away from the discussion. But the fact is that there are lots of values we all share. Nobody argues that discrimination is morally appropriate. A lot of people, unfortunately, engage in it.
Nobody argues that lying is better than truth-telling.
Nobody's saying it's appropriate to engage in sexual harassment. There are lots of values that you can transmit before you get to the argument about whose values you're going to teach. Yes, but some teachers are afraid to teach values because parents in their communities are vocally opposed to schools' taking on that role. The community does not agree on common grounds. These are the exceptions to the rule.