Um aluno não pode assistir a uma sessão mas continua no grupo — como?

Click the session, then untick that pupil in the popup. You’ll be asked whether it applies to just this session, or to all sessions of that subject this term for the group. Either way they stay a member of the group and attend everything else, and it updates their app and the register. Re-tick to reverse it. (If instead they’re simply not in on a whole weekday, use the group drill-down from the row name.)