If drives are identical in all servers, why don't you just randomly mix them? Pull drive 2 from server 1 and swap it with drive 4 from server 2, etc. Then just destroy the arrays, create new, preferably different RAID levels and just write some sample data.
Wouldn't this leave quite a risk of the data being on a drive still?
I have mixed the drives. Destroyed the arrays, and set as Raid0. Then, running shred on those new Raid 0 arrays...
Risk? Unlikely. If you mix few drives from each array in few servers, there's no way to recover it unless you get the original set of drives together. The more drives and servers, the lower the chances of re-assembling the array. You're not donating these to NSA, are you?
No, lol. Two servers are going to a School to be their production environment. Another server is going to a different School to be a lab machine so students can try virtualisation.
Then just make sure complete set of disks from any server doesn't end up in one school. Schools don't have the budgets/personnel/skills/time/motivation to play the NSA.
Let me illustrate what will happen when you mix disks. In a set of 6 disks in 3 servers you have some data, but that data is completely unknown to bad actor. So:
ABCDEF - in server 1, abcdef in server 2, and 123456 in server 3. After mixing you end up with Ae2DE4 in server 1, a3BF16 in server 2 and bcCd5f in server 3. After writing some random data you'll have Ae2DEx, a3BF1y, and bcCd5z. Now go ahead and try to recover original data, not knowing what it was in first place. And do it on school's time and budget.