I was sad...

A few days ago, we were writing the exams at our school.
Though I passed, but I was just (only) the 52nd in our grade...
All the / my grades were really bad, because they dropped lowered everyone's/my performance average.

I have renewed my/the efforts to improve. I can't mustn't be lazy!!

Refuel !

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1) "our school had the exams" - In Eglish, it is not possible to state that a school has something. ;) And there is not the before exams, because they weren't mentioned before (Singular - the / a/an | Plural - the / -nothing-)
2) "Though...,but" - There is no need for but when you use though. You can either use though, which by itself excludes the next part of the sentence or you can use but on its own. ;)
3) "I was just 52nd" - I don't really know why, but just is not used in this context. You can use only to express it's low or you don't have to put anything there.
4) "The grades were really bad" "dropped many performance" - It wasn't very clear whether you meant that all the grades were bad (everyone's) or just yours. And you can't use many performance. ;)
5) "I can't be lazy!" - This would mean you try to be lazy. You have to use mustn't, which expresses that someone else or you yourself forbade you.
6) "Refuel!" - I suppose you meant the Chinese phrase 加油. Its literal translation really is refuel, but it is just a phrase, which doesn't have any equivalent in English. You could use "I have to be strong" or something like that, but it doesn't have such a strong meaning in English, so I would just leave it.

:)