Hand the tablet over and let them tap their own name. No email, no typing a username.
Set one password you can remember and give every pupil the same one — you are not remembering thirty. Set it when you add them, or change any of them later on your account page.
Keep this screen handy: in Safari tap Share, then Add to Home Screen. It then opens like an app on the tablet.
Each pupil sees only their own work. Tapping a name fills their username in — they type the password and they are straight into their own activities at their own working level.