Utah is also only representative of about what, 6% of the entire nation's population? Lets do the same in all of NY, California, and Texas for a few years and the chance of a problem arising will increase exponentially. Taking your argument, I can say that school shootings are not a big deal because I don't remember any mass school shootings in Hawaii.
Additionally, regardless of what your position, you MUST agree that having guns in school will increase the likelihood of a student being shot in that school. The chance may be large, or it may be small, and thus worth taking. However, it is indisputable that the risk, however minimal, is there. So again, taking your Utah example, as far as I'm aware, there has not been a single incident in which a school shooter was stopped by an armed teacher. So if no measurable benefit has been realized, why accept the marginal risk of having guns in schools ?