/Gotcha.
At this time the only way to apply group policy to specific users would be though Active Directory. /:
That being said, you might be able to make the process a lot less time-consuming with one of the following options:
1) Use Sentry Policy (SM enrolled devices only): If your devices are enrolled in SM you can give users specific tags and apply group policy to an SM tag. Again, this would require SM enrollment for the devices in question [Relevant KB]
2) Apply Policy to Specific VLANs: If you use separate SSIDs and VLANs for the users you want to have on a schedule, you can apply the group policy to the entire VLAN rather than user-by-user. For example, having a Teachers/Faculty VLAN and a Students/Visitors VLAN means you can apply scheduling to the Students/Visitors VLAN without having to apply the policy to all students manually. [Relevant KB]
3) Apply Policy by Device Type: If all students are given Chromebooks (for example) and you want to limit student access at specific times, you can apply group policy to any Chromebook that connects. [Relevant KB]
The viability of these options varies depending on what devices/users you are looking to regulate.