There are two types of Group Page Access for each group type, School Level and Individual Access. If access was added to the School Level and it still is not working, it may be because there is Custom Group Page Access for that group.

To check if there is Custom Group Page Access:
  1. Navigate to Core > Security > Group Page Access > Select the appropriate group type (such as Classes, Activities etc.)
  2. Depending on which group type you selected, select Individual "Group Name" (for example Individual ClassesIndividual Activities etc.)
  3. From the drop down to the right right of School Level, that is titled based on the group type such as Individual Classes, locate the group, but do not select it (as that would actually add the (C) custom setting.
If the group title ends in (C), this means there is custom access set within this group. You can either add the Role(s) to have access to the Custom Access, by selecting the group and setting up Group Page Access. Or alternatively, you can select Remove Custom Access, which will remove Custom Access for that group. 
Remove Custom Access

Then, it will follow the default settings as found within the School Levels Button within that Group Type:

Group Page Access 2