Ordinary clothes will do. Don't be too casual, it will give people a frivolous feeling; But don't be too formal, it will give people a feeling of over-fabrication. If the student union has a uniform, wear it. If not, just plain monochrome trousers and a timely shirt (don't be bright, try to be monochrome, and consider avoiding pure white shirts). If it is autumn and winter, you can add an extra coat (not too fancy, try to be close to the color of pants)