Office 365 Data rentention

[alfatec]

Just updating our GDPR Data retention schedule and need trying to find some information on Office 365 email retention periods. Issue We have is that all staff and students have Office 365 email but when they leave the school the account is automatically removed from Office 365 after 30 days. So unless we leave all the accounts in Active Directory for ever their emails will go 30 days after they leave. What do other schools do for email retention with office 365??