I disagree.
The “uniform of the day” is cap and gown. There is no requirement or prohibition for what is worn underneath. Unless USMC regulations prohibit wearing blues under the gons, with the cap replacing the normal uniform headgear due to its being covered, the school is correct.
I’m a former Army O-3.
Army Zero... Got it.
Go through Marine boot camp and get back to me about our Dress uniform.
Those of us who are proud of our service refer to that rank as "captain".