@here we now have a solution for this (it came from an idea on this community forum!)
To enhance the security of your Opal sessions and prevent the disabling of Opal’s Screen Time during a session, follow these steps. This process should take less than 5 minutes to set up.
https://www.opal.so/help/how-to-lock-opals-screen-time-access
Please be aware that you won’t be able to access your phone’s Settings while in a session (unless you’re on a break).