Hello @SAitken ,
Thanks for reaching out.
It must work with both account types like: Organization and school accounts, If Azure portal says "You have already activated this trial before" which mean respective Azure AD tenant might have already utilized trial Azure AD PREMIUM P2 license by other users who are part of same Azure AD directory in past so Trial license can't be used more than once.
Alternative, you could try using EMS( ENTERPRISE MOBILITY + SECURITY E5) trial from Azure portal this EMS plan includes "Azure AD Premium P2" feature as well as. Hopefully, this way you could continue exploring Azure AD P2 features and EMS E5 trial includes 250 licenses and will be active for 90 days beginning on the activation date.
If non of these works for you then I would recommend you to create new Azure AD tenant and avail free trial subscription to explore Azure AD features.
Hope this helps.
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