Media limits are applied at the account level and not the Class level or group level. Hence by deleting the class will not provide you more storage space. The system counts the number of media items you uploaded. Hence to be able to upload more you will have to delete the media items from the right place. 

Please delete media to stay within the 125 item limit. Here are the steps to delete -

  1. Click on <Show For All Groups > in the left menu
  2. Click on the Media tab,
  3. Click Albums
  4. Click "Media Added by You" album
  5. Select enough items in the "Media Added By You" album and remove