The standard form of the argument exhibits the flaw quite clearly: 

If better teachers is tenured(A), they are politically conservative(B)—(Premise)

and none of the better teachers are tenured(not-A)—(Premise)

Therefore the better teachers are not conservative(not-B)—(Conclusion)

Attack:The above argument commits the "denying the antecedent" fallacy. The arguer fails to recognize that being tenured is not the only reason that could make better teachers politically conservative. As a matter of fact, there are other reasons that are sufficient to bring about being conservative in addition to becoming better teachers. Therefore, the form of the argument is structually flawed for it aims to deny the consequent without realizing the antecedent is not necessary.