When a school's dress code is used to ban clothes because they have certain slogans, then it really becomes an issue of free speech and not just personal appearance. The school should not ban any slogans unless they are libelous, obscene or somehow very disruptive.
Some schools try to use their dress code to prohibit students from wearing T-shirts which advertise certain beer companies. The school officials argue that these T-shirts encourage drinking and since underage drinking is prohibited by state law and school rules, the students shouldn't be able to wear the shirts. The problem with this argument is that it doesn't make the distinction between speech (having a beer slogan on a T-shirt) and conduct (students drinking beer). When they argue that wearing such T-shirts is the same as actually drinking beer, they ignore the basic principles of freedom of speech. The problems of drinking at schools should be dealt with through education and counseling, and not by censorship.
Can the school make me wear a school uniform?
Although schools are allowed to adopt school uniform policies, there are certain rules that schools must follow in doing so. One of the most important rules is that schools must allow parents to opt out of having their children wear school uniforms. Parents may opt out of the uniform policy for any reason, or no reason at all. The law does not permit the school to decide whether the reason is good enough. Nor can the school discriminate against students who do not participate in the school uniform policy or provide rewards to students who do wear uniforms, such as special treats for the class with the largest number of students wearing uniforms. In addition, schools that have adopted uniform policies must have in place a program to assure that students whose families cannot afford to buy uniforms are able to get uniforms if they want them.