Why did the Egyptians who graduated from school wear white clothes and did not have calluses on their hands?

If they finished school, then they knew the writing. This meant that they would not need to work as manual labor. Such Egyptians were mostly from the nobility. They occupied the highest paid positions, since at that time the profession of a scribe in Egypt was in great demand. Thus, such Egyptians wore clean and white clothes because they did not work in a construction site or in the field.