You can't do that though, if you are re-installing Windows, with an off the shelf OEM Windows 7 DVD? You could leave the factory installed windows installed and modify the installation. Un-installing what you don't want and adding what you do want. It sounds like the OP wants to start with a clean install and go from there though.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Home Built
OS
Windows 10 Education 64 bit
CPU
AMD Phenom II X4 980 Black Edition Deneb 3.7GHz
Motherboard
Asus M4N68T-M V2 µATX Motherboard
Memory
8GB 4GBx2 Kingston PC10600 DDR3 1333 Memory
Graphics Card(s)
Zotac NVIDIA Geforce GT640 2 Gig DDR3 PCIe
Sound Card
VIA VT1708s High Definition Audio 8-channel Onboard
OK, but if its SLP media the OP shouldn't be prompted for or have to enter a product code at all during the install? He stated he is entering the product code from the COA sticker when asked for a product code, so I assumed it was just a regular OEM DVD. If it is SLP media though you're right, there should be no need to use the change product code option. They can all use the master SLP product code.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Home Built
OS
Windows 10 Education 64 bit
CPU
AMD Phenom II X4 980 Black Edition Deneb 3.7GHz
Motherboard
Asus M4N68T-M V2 µATX Motherboard
Memory
8GB 4GBx2 Kingston PC10600 DDR3 1333 Memory
Graphics Card(s)
Zotac NVIDIA Geforce GT640 2 Gig DDR3 PCIe
Sound Card
VIA VT1708s High Definition Audio 8-channel Onboard
1) Startup one new PC's, doing all the new install bits.
2) Setup personal setting.
3) Removing the OEM-SLP Product Key.This bit is the big problem. 4) Cloning the HD Drive to use on the other 35 new PC's.