Hello,
K-12 schools, or locations that offer services to those under the age of 18, are not allowed. Crossing signs would also be considered indistinct, as many schools and other neighborhoods have them. More info can be found here:
Lastly, you are not submitting things to be stops or gyms, but Wayspots. Approved Wayspots get added to the Lightship map. Lightship is used by Niantic games and other developers; Niantic uses them mainly for game play locations (Power Spots excluded). Each Niantic game has its own density rule for game play locations, with PoGo using S2 cells and Ingress using a 20 meter rule. More info on S2 cells can be found here:
https://pokemongohub.net/post/featured/comprehensive-guide-s2-cells-pokemon-go/