Gentlemen are provided with a School Suit, Ladies are provided with a Blazer. They were designed and implemented by Nanahoshi aka "Silent". They look like Japanese student uniforms.
The Gym uniform is a robe and the academy doesn't provide them nor do they specify any restriction or preferences on what to wear. So students can buy whatever they want and for the case of Rudeus, he uses the robe he had been using from the Demon Continent.