I think the activity log only lasts a few months, ours shows no activity even when we use the accounts the activity was made on...
I had a similar issue today, where the original CSVs were not left by the previous tech and the accounts were created with some locations having an apostrophe and others not.
I called the customer support number from the Apple website, then after 15 minutes on hold, I was eventually given the number for Apple Education Support (0800 912 7302), who mainly deal with ASM account support...
The Apple Education Support eventually forwarded me to Apple Enterprise Support who do not have a direct line, you have to be redirected.
They told me that the Apple School Manager does not have any way to export data and if we do not have the original CSVs they would need to be manually recreated, but some MDMs do offer ways to export the data they get from ASM which will get me started.
Our MDM is Zuludesk and can export users, but do not have all the required fields for ASM