"Suspended HS students take stand against censorship of Confederate Flag"
Wrong.
The students take a stand only to exhibit their ignorance of the law, as does the OP.
Schools prohibiting the wearing of shirts with a 'confederate' flag does not constitute 'censorship,' nor does it violate the First Amendment.
In Hazelwood School District v. Kuhlmeier the Supreme Court held that the “[f]irst Amendment rights of students in the public schools are not automatically coextensive with the rights of adults in other settings, and must be applied in light of the special characteristics of the school environment. A school need not tolerate student speech that is inconsistent with its basic educational mission, even though the government could not censor similar speech outside the school.”
Clearly a flag used for decades to advocate for racism, hate, ignorance, and violence is “inconsistent with [a school's] basic educational mission,” which is to disseminate the values and mores of a free and democratic society, values and mores undoubtedly in conflict with what the flag represents.
And that the students might subjectively perceive the flag to not be racist, or a 'symbol of history' is irrelevant, where schools are authorized to make that final determination, and prohibit the speech in accordance with the Constitution.