Student data, especially grades, must be treated confidentially. Do not use external services to store or process this data. The department provides NextCloud as an alternative.
The following is a quote from Prof. Peter Müller:
I wanted to remind you that student data, especially grades, needs to be treated confidentially. Please do not use any cloud providers to manage your courses and exams. We have various solutions in place to share data within the department and ETH. So please don't put student data into Google spreadsheets, on Dropbox, into Gmail attachments, etc.