If you use Google Classroom, we suggest the following as easiest.
Assign our CS+Earth Hyperdoc (student-facing googledoc). Just one for each week.
Students access starter code and helpful materials from the Hyperdoc.
Before closing their computer, students save their Scratch projects to their computer (see below) and upload to a googledrive.
Students paste a link to their project on googledrive in the Hyperdoc form.
There are many other processes that will work, but this is our recommended one.
We strongly recommend you have students use the Scratch website, but do not login (or create accounts). When students click on our starter projects, they will open on the Scratch website. They then modify them and BEFORE they close their computer, download them to their chromebook/laptop.
You will need to follow this process for each student project (assuming they sent you a googledrive link where the .sb3 project is stored).
Download the student .sb3 file to your computer.
Open the Scratch web site in Create/Edit mode.
At the top, click on File->Load from your computer.
You can now play/explore the student's project.
Your students will need to upload a Scratch project file (extension is .sb3) someplace you can access it. You have options, which may be based on your LMS. Some examples:
Create an assignment where students are directed to upload their .sb3 file to a googlefolder and submit the link.
Set up a googledoc shared with the class with a table where each student copies in a link to their uploaded .sb3 file.
Just take the day off! Occasionally the website gets broken or needs maintenance. Don't worry. Read a book and come back tomorrow.