I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer.
Issue:
You were trying to disable a guest account that you created via the "Let guests sign into these PCs" option in" Set up School PCs." You tried using the OMA-URI - Accounts_EnableGuestAccountStatus option, but were not able to disable the account.
Solution:
You had to reference this OMA-URI:
./Vendor/MSFT/SharedPC/AccountModel and set it to "1."
Thank you again for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.