Question

how to manage time when you are in class 8 ​

Answer 1

Actually 8 is the last year you can completely enjoy.
To manage time,
Studies- practice maths everyday and do one or two more subjects when there are no exams (2or3 hours minimum) these hours should be full of concentration so that you do not have to waste more time. You can study for more than 3 hours if you wish.
You should have a good 8 hour sleep.
During exams try to study most of the day and avoid phone and other devices.
Once the exams are over you will be really happy to get these things back so have patience.