Just a page with a form you can upload files through. You can setup something like that e.g. with Owncloud/Nextcloud. And dedicated education tools also often have it.

For normal document-sized files, email is fine though too IMHO, for university stuff I never really cared which way was used.

NextCloud is quite a dependency for a simple file upload.

Sure, if you just install it for that. But it's a reasonably common service (e.g. around here, many universities have it for their employees and students) with that feature.