1. It's not my study. It's over 50 studies in the extant scientific literature.
2. If the image of a gun increases the level of violent actions, as the studies show, then having that image on a shirt that is seen by a class full of students (or multiple classes of students), then does it not make sense that seeing the imagery of a gun would increase violent actions? So, it is not just portraying violence, it is, in fact, leading to an increase in violent behavior.
3. Regarding your analogy: I do not know the literature on this as well as I know the literature on the gun issue. The studies I do know suggest that the material has to be sexually explicit to lead to outcomes that are generally considered negative (early activity, casual and/or unprotected activity). So, unless the imagery were explicit, I do not see why it would be a concern for the school.