Clean school makes us feel interested in going to school. Students and staff can breathe nice and fresh air. So the students will not fall sick because of diseases. There will not be mosquitoes or harmful organisms nearby.
Further students and staff will feel more happy to live in a clean building and clean surroundings. Imagine foul smells coming out from nearby garbage can. Children cannot breathe comfortably. So the pupils cannot concentrate properly on learning or teachers cannot focus on what they are teaching.
Since there are a number of children together in a small class room, fresh air is a must. When the walls, chairs, tables, compound, toilets etc. are all clean the mind of the children feel nice. If the toilets are not clean, then children feel very reluctant to go to the toilets. That gives them a bad impression about the staff and school management.
When the school and its surroundings are clean, the reputation of the school also goes up. Students learn to follow discipline and will inculcate habit of cleanliness. The will learn more civilized culture.