It does violate the constitution. Freedom of speech and expression is limited if it threatens, violates, or causes substantial harm the rights of one group or person.
Also, when you're in a school building, you must abide by the laws set by the school itself, which usually voids fourth amendment rights, and many others.
update: Tell your cocky friend he's a dumbass, thinking about suing the school because the administration asked him to flip his shirt. I have a friend that literally left his backpack in an office, and they searched it and found a machete, then suspended him for a week. That would be a better reason to file a lawsuit, but being asked to flip his shirt is ridiculous.