How to completely remove data from school iMacs?

hello.

i'm donating my perfectly functional iMac to a school.

Does restoring an iMac to factory settings count as having all previous data removed under the WEEE UK standards?

If not, how do I remove all traces of personal user data etc from the iMac?

I don't think they came with disks and I believe they are 3 to 5 years old.

Thanks in advance.