IOS – Educational Volume Purchase and B2B (iOS apps)

Here is the relevant excerpt of the iTunes Store Terms and Conditions:

https://www.apple.com/legal/internet-services/itunes/volume/us/terms.html#VOLUME

CONTENT CODES

[...] YOU UNDERSTAND AND AGREE THAT THE OWNER OF THE REDEEMING ITUNES ACCOUNT WILL BECOME (i) FOR AN APP STORE PRODUCT, THE LICENSEE OF THAT COPY OF THE PRODUCT; AND (ii) FOR AN IBOOKS STORE PRODUCT, THE OWNER OF THAT COPY OF THE PRODUCT, AND SHALL BE ENTITLED TO ALL ASSOCIATED RIGHTS. SUCH LICENSES OR PRODUCTS ARE NON-TRANSFERABLE.

for short: only managed distribution retains ownership!

If your app is in fact targeting both employees and people external to the company, then the Public App Store is your only choice anyway. All other options like VPP or the Apple Developer Enterprise Program do only allow you to distribute apps to employees. So your argumentation towards Apple would be that it needs to be the public store, because there is no other way to distribute that app to a potentially global audience.

However they might not like your authentication method "unique code", as Apple usually wants apps to have a signup method and a more sophisticated login, such as username and password.

Adding some external (publicly visible) content near the login might help to get the App approved as well.