Wipe iMac A1311 Firmware Password?

I have bought and sold several off-lease iMacs from local universities in the past. I've always been able to just wipe the PRAM with command+option+p+r and then go on my way.

With these newer iMacs, this apparently isn't an option. I've got 3 iMac A1311's with firmware passwords. I can't get them to boot to anything, nor can I boot by pressing "option" since it just shows me a lock, indicating it has a password.

Is there a way past this without going into an Apple store? I have the receipt and I can show I bought it from the university, but this is going to be a major pain since the nearest Apple store is an hour away from me and I've got 3 with this same issue.

Thanks in advance.