As for cancelling the subscription, all you need to do is to go to your account page and cancel it.
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Here is how:

#1 From any of your devices:

Just follow the steps in the below link
From Apple:

https://support.apple.com/en-eg/HT202039

From Android:
​https://support.google.com/googleplay/answer/7018481?co=GENIE.Platform%3DAndroid&hl=en
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OR

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#2 From TeacherKit application:

1. Open TeacherKit account

2. Tap on the ”Account" icon in the top of the sidebar of the classes view

3. After opening the account screen, scroll down and you will find "Cancel Premium" button. just tap on it. 

4. Follow the steps that will appear to you after tapping "Cancel Premium".