can't restore deleted files because storage/bin is full with said files.

Make sure you are signed into OneDrive at the blue cloud icon.  Move files from your PC's OneDrive folder at C:\Users\username\"OneDrive for school" to your external drive.  Do not move files to the Desktop, Documents, or Pictures folders if those folders are backing up to OneDrive.  

At the very least, you will rescue some files and free up space in your OneDrive.

Then try selecting the first file in the Recycle Bin.  Hold down the Shift key, scroll to the last file in the Recycle Bin, and select it.  This should select all files in the Recycle Bin.  Now click on "Restore" and give it a few hours.