The U.S. Supreme Court in Tinker v. Des Moines (1969), famously said "Students do not shed their constitutional rights when they enter the schoolhouse door."
The Constitutionality of the school principals action entirely hinges on the standard of whether the shirt would unreasonably disrupt normal school functions. Although the U.S. Supreme court has not revisited the issue of Political Free Speech in Schools since the Tinker decision, a number of lower courts have addressed the issue in cases similar to this.
In Castorina v. Madison County School Board the 6th Circuit held that wearing a shirt with a Confederate Battle Flag printed on it to school was protected political free speech.
In a U.S. District Court Case from the Southern District of Ohio which was not appealed, Nixon v. Northern Local School District Board of Education, the Court prohibited a school from preventing a student from wearing to school a T-shirt with a Biblical passage and the statements “homosexuality is a sin, Islam is a lie, abortion is murder, some issues are just black and white” printed on it.
Even more closely yet to the Obama T-shirt case. In a case from Vermont Guiles ex rel. Guiles v. Marineau, the 2nd Circuit (and denied certiorari) held that a boy, who was thirteen at the time the case was brought, had the right to wear a T-shirt to school on which were printed statements critical of President Bush such as “Chicken-Hawk-in-Chief” and stating that the President was on a “World Domination Tour.”
Humm, how do you think the principal will try to distinguish what he did from Guiles? I believe the Dalton boy and his family have a very good chance of teaching the Aurora, Colorado School District some lessons about Political Free Speach.