This summer will be refreshing vast majority of student devices.
I also have a number of old device records within Jamf (not known if they were removed from Apple School Manager).
What are some recommended workflows regarding devices taken out of service and removed?
I suppose it depends on a lot of things. But we have a couple of processes that seem to work well for us. I am assuming that you mean the technical side of the workflow and not fleet planning and acquisition? I'd be happy to talk about that if you're interested. I find it important as we stagger purchases across the fleet. We REALLY try to avoid spikes in purchasing (not buying for a few years and then refreshing all at once). Besides, it usually helps the CFO budget... or at least accept our budget. As for the computers themselves: the short version is that we gather up all the devices that are being transitioned out, collect their serial numbers (We barcode everything so it's pretty quick), and use this as a check against whatever policy/list we've used to determine what are leaving. Those units are released from Apple School Manager (just based on the list of serial numbers). There's nothing wrong with trying to release a serial number a few times because as they can only be released once, but that won't fail the process if you're using a csv to release them. Then. in the JSS, they are set to a department that we assign to devices ready to transition out. That enables a policy that wipes the machines flat (see the one-button restore discussions). How they get removed from the JSS depends on a few things. Sometimes we have the machines remove themselves, and sometimes we do it manually.
So, I know this is rather vague. If you've got specific questions I'd be happy to give any insight I can. We only move about 100 DEP devices a year. BYOD is about 350 in and out, but that process is slightly different.