Parking. Off-street parking shall be provided in the following ratios. Nursery and elementary schools shall provide one parking space for each staff member or employee, plus one parking space for each 10 pupils, plus adequate space for buses and delivery vehicles used in conjunction with the activities of the school. Intermediate and high schools shall provide one parking space for each staff member or employee, plus one parking space for each five pupils, plus adequate space for buses and delivery vehicles used in conjunction with the activities of the school. All such cars, buses and delivery vehicles must be garaged during the nighttime, except that noncommercially registered school buses and vans may be stored outside at any time, provided that they are adequately screened from the street and adjoining property owners in a manner satisfactory to the Planning Board. These requirements may be increased if, in the judgment of the Planning Board, such considerations as the unavailability of bus service or the distance from centers of population or a relatively high percentage of pupils driving their own cars make such increased requirements desirable and necessary to lessen congestion in the area.