Student was not likely to succeed on merits of his claim that high school's refusal to permit him to wear T-shirt displaying images of automatic handgun and purporting to be hunting license for terrorists violated his First Amendment free speech rights, and thus student was not entitled to preliminary injunction barring school from enforcing ban, despite student's contentions that T-shirt was intended to show his support for United States troops in Iraq, and that T-shirt reflected federal “Rewards for Justice” program; language on T-shirt advocated use of force, violence, and violation of law in form of illegal vigilante behavior and hunting and killing of human beings, and Rewards for Justice program did not authorize non-governmental personnel to hunt or kill suspected terrorists