You asked

How do I get my school child to do what I ask?

Step 1: Give forewarning

You need to say: " In five minutes, I need you to stop playing that computer game and get dressed for bed. This is your five minute warning".
Step 2: It's Time

You should say "Now it is time to get dressed for bed. Please turn off that computer game and you can play tomorrow."
Step 3: Deliver the threat, just once

You say: "If you stop, wash up and get dressed for bed, then you read for ten minutes before bed, if not your light is being turned straight off." Whatever consequence you choose, you need to state it clearly and calmly and you need to show that you are prepared to follow through if she doesn't comply.
Step 4: Follow through with your threat

This is when you follow through with your threat. There is no going back now or no room for discussion. Discipline your child and ensure that you follow through.

You should say: "I am switching this game off, change into your pyjamas, wash up and brush your teeth. I want you to get straight into bed and turn off the lights". 

At this stage, you will most likely experience a huge tantrum or meltdown. Yes it's unfortunate but it will only happen once or twice more as your child will now have learned that you mean what you say. 

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