Grammar review... please check it!

2. The tallest in the classroom is the cleverest too.
The tallest person in the classrom is also the most clever

3. Walking alone could be very sorrowful, but also useful to reflect on (to think?).
Walking alone could be very sorrowful, but could also help you reflect

4. He asked me what time the bus would have arrived.
Correct, but this means the bus didn't arrive by some reason. If you want to know when the bus will arrive, you say
He asked me what time the bus would arrive

5. I'd never want to take an airplane: it scares me very much.
This means you wouldn't want to steal an airplane
I'd never want to go on an airplane, I'm too afraid something will happen.

6. If I had the possibility, I would leave to New York immediately.
If I had the opportunity, I would leave to New York immediately

7. Big cities don't excite me, I would rather small villages.
When writing like that, it seems "small villages" is a verb, which is not correct
I'm not very fond of big cities, I prefer small villages

8. If I had told all to John, he would have never forgiven me.
If i had told John everything, he would have never forigven me

9. I really like going with my family to take an ice cream and go around in town.
I really enjoy goining out with my family to grab an ice crean and walk around town together

10. If it starts raining, we'll all go to the bar around the corner.
Correct

11. I've bought this T-shirt at the shop in front of the school where my brother attends the 4th year of primary.
Here there might is a misunderstanding, it seems like your brother attends school in the shop
I've bought this t-shirt at the shop in front of the school. My brother attends the 4th year of primary at that school.

12. If he hadn't helped me, I would have not known what to do.
If he hadn't helped me, I would not have known what to do

There you go! :)