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"On the one hand, he is very accomplished in all the subjects he takes in the school compared to the rest of his class, but on the other hand, when it comes to usual things that require manual dexterity, he fails miserably."

"We were promised to go on unforgettable vacation with our school, however, all of a sudden, our headmaster had an abrupt change of heart and though most of teachers were in favour of such an inviting prospect of being somewhere else than in the dull school grounds, the excursion was unconditionally cancelled."

"I insist that you go with me for a drink to celebrate our roaring success in the school-leaving exams."

"It wasn't until the evening that I reflected on the implications of a decision that still lay ahead of me."

Thanks in advance!
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Here are my versions. You can compare them with yours to see what I regard as improvements. Emotion: smile
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On the one hand, compared to the rest of his class he is very accomplished in all his school subjects; on the other hand, he fails miserably when it comes to ordinary tasks that require manual dexterity.

We were promised an unforgettable outing from school; however, our headmaster had an abrupt change of heart, and though most of the teachers were in favour of the inviting prospect of being anywhere but the dull school grounds, the excursion was cancelled.

I insist that you join me in a drink to celebrate our roaring success on the school-leaving exams.
[ I assume that "school-leaving" is a legitimate term in BrE. AmE: "on the final exams." ]

It wasn't until evening that I reflected on the implications of the decision that still lay ahead of me.

CJ
Comments  
Thank you very much CJ, your improvements make my sentences now absolutely perfect! Emotion: smile