Student's file not saving, used the download feature and now file is not opening
Suggestions?

if you downloaded the .sb3 file, you'll have to use scratch 3.0 to open the file. you can't open it with other apps.
and in case it isn't obvious, downloading the file also doesn't update the cloud-saved version of the project. to do that, you can overwrite the project with another .sb3 using ‘load from your computer’ in the file menu in the editor.
however if the project is not saving, you could try importing the .sb3 into this tool to see if any assets in the project exceed any of the limits (scratch doesn't like assets that exceed 10mb)

TheGlassPenguin wrote:

if you downloaded the .sb3 file, you'll have to use scratch 3.0 to open the file.

This is correct. Additionally, you can't open an SB3 file by double clicking it; you have to go into the Scratch app and click “File > Load from your computer”.