You can enable a school mode and set the time multiplier to "Free usage."
This ensures that any session they start will be multiplied by zero, meaning it won't consume any of their free time or earned token time.
You can do this with the following steps:
1. Login a Parent User
2. Go to Manage Modes - Create a Mode - Name it "School Mode"
3. Choose a picture for the mode and turn off "Allow Concurrency"
4. Turn it on for the child users who can use the mode
5. Set up time for School Mode - For example: Monday to Friday, 8 am to 4 pm
6. Tap on Save
7. Now tap on the "School Mode"
8. Go to Daily Limits Tab
9. Select drop down for the Time Multiplier - Choose Free Usage