In many schools, certainly here in my area of California, wearing any sports team logo at school is forbidden.
This is not because it is intrinsically distracting, but for a much sadder reason. Unfortunately the gangs have selected sports teams as their identifying objects. Wearing red or blue "rags", shirts, bandanas, and many of the other ways the Crips and Bloods try to identify themselves has been banned, so now the Crips wear Raiders gear and the Bloods wear 49ers gear.
In our school district, a portrait of a notable athlete would probably fall under the "gang related" ban. This is something the gangs have forced on the school administration.
Unfortunately when you make such a rule it has to be applied school wide. Therefore, IF his district faces such a problem and some local gang claims the Spurs as their identification it would be a valid rule for the school to make.
This is just speculation but serves to show some reasons that certain schools have peculiar rules that do not make sense on their face, but are in fact important to preservation of school order.
The particular article quoted should be labeled an editorial, as it makes no attempt to investigate and report the facts but simply tries to make jokes of the subject.