Instead of tagging areas of the school as 'landuse=grass' recommended practice is to use 'landuse=recreation ground' and 'surface=grass' as per https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dschool#Single_school_on_one_site.  

Also instead of drawing multiple adjacent areas of the same type it is good practice to create a multipolygon and and cut out any different areas by making them `role=inner`