Spore on Mac - This code has already been used.

Hello. I'm a teacher. For years I've been using Spore with my students for a project. The kids love it. 

In 2013, I purchased multiple (six) copies of the game from GameTree as a digital download. I downloaded the ISOs and installed them with the serial numbers provided by GameTree. And it's always worked. Over 9 years, I've had to reinstall several times as the school district has wiped the six Macs I have in my classroom. This year, I've gone to reinstall, I've entered my serial numbers, and I get the "This code has already been used" message from EA's servers. 

GameTree is now out of business, but I have the email from them confirming the purchases. 

The Macs are 2011 iMacs running High Sierra, so they can run Spore fine. It's just the issue with the serials. They will NOT run Origin. I've tried - the school's firewall blocks it. I need to use the digital downloads I bought and install it the old fashioned way. 

I've already spent hours and hours talking with EA support - via phone, chat, and email - with no luck. 

How do I get Spore running for my students?

Any help would be very appreciated!!

C.D.