What does the First Amendment actually say and NOT that modern clap-trap?
Congress shall make no law abridging the freedom of speech.
There it is verbatim, minus ‘the claptrap’, which would be modern case law to others.
The Principal was wrong. Period. As written, you can actually shout ‘fire’ in a burning building, but that seemed recklessly dangerous and arbitrary.
To recap - the Principal was wrong, should apologize, and should know the law a bit better before he goes ordering students to ‘shed clothing’.