We have recently noticed a problem with exporting projects to Google Classroom on iOS. The exported files are not correctly attached and instead, are shown as a link to a local file that looks like: "file:///private/var/mobile...".
Unfortunately, we're unable to fix this on our end as it appears to be a problem with Google Classroom and affects many other apps as well. There has been a bug report submitted to Google that we have also added to in the hopes of helping them resolve this issue in a timely manner.
As a workaround, we can only suggest exporting a project to Google Drive and then, from within the Classroom app, attaching the file to an assignment. Please note that sending a file directly from Google Drive to the Classroom app does not work either.