With a Socrative paid plan, you can create subfolders. Here's how:

Navigate to the Library tab. Once there, select the folder you'd like to create a subfolder in and click "Add" on the right. From the drop-down menu, select "New Folder":

A menu will appear, and you can give your subfolder a name. Then, click "Create":

Your new subfolder will then appear at the top of the page within your folder. You can reorganize it if you'd like!