Update grade-school tests

[Elahi-cs]

This PR aims to update all tests in the grade-school exercise. The tests were created manually because the fact that some object-oriented adjustments were necessary made it impossible for the exercise_creation_helper to actually help. I'm hesitant the changes were done correctly because of my lack of experience with the platform, but I've tried to stick to the original test cases where possible and to use the same style with the missing test cases.

@vaeng let me know what you think.

[github-actions]

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[no important files changed]

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[github-actions]

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