We've implemented some hooks to help educators out with device unlocking. If you're using device configurator software, there should be an option to unlock Assembly. Unfortunately there are too many configurators available for us to test all of them so this may not always work.
Versions of Assembly purchased through Apple's "Volume Purchase Program" should unlock automatically.
Although we wish we could, we cannot provide promo codes to unlock Assembly on multiple devices because Apple limits how many we can use. Always feel free to reach out to us if you wish to discuss this in more detail.