Off-street parking shall be provided in the following ratios: 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. All other 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. These requirements may be increased if, in the judgment of the Planning Board, such consideration as the unavailability of the bus services, the distance from centers of population, or a relatively high percentage of pupils driving their own cars make such increased requirements desirable.