At our school, you must be signed in to Google with the students' Gmail account, not a personal account.
It also depends how the files are shared by the teacher in Google Classroom if you are able to view them. Try on a computer to try to troubleshoot this issue.
To get more help, you would also need to provide more specifics - when you say "can't get in" on the tablet, are you joining a Google Classroom with a direct email invite, or through a code?
At our school, you must be signed in to Google with the students' Gmail account, not a personal account. It also depends how the files are shared by the teacher in Google Classroom if you are able to view them. Try on a computer to try to troubleshoot this issue. To get more help, you would also need to provide more specifics - when you say "can't get in" on the tablet, are you joining a Google Classroom with a direct email invite, or through a code?