The echo of the class hasn't of laughs hasn't stopped so quickly. (I don't understand what you mean to say here. You doubled your negatives and your perfect tenses, so the sentence is very confusing.) Even we two, plus our teacher, has laughed but we felt a little embarrassed. I realized that I was lucky to not get a bad mark for helping her this way.

Because you're telling a story about the past, you will never need present perfect tense. Never.

I don't know if this "story" can be the same as told by me, through writing and in English but I have tried to write about something funny because I remember that very often about that. I told her I would never forget this as long as I lived my whole life. I hope I can respect my "vow".

Here you are talking about the present, so the present perfect is OK.

Perhaps the key phrase for how you told the story is conveyed accurately (or not?).  I got the idea, so don't think that anything was lost in translation. :)

I told her I would - reported speech grammar.

This reminds me of a lovely scene in Cinema Paradiso: http://www.youtube.com/watch?v=UK4ilBiWCCA

The teacher is drilling a weak student on the times table. She asks, "quanto fa cinque per cinque?" (How much is five times five?)  The student answers with  quarante (fifteen), but obviously this is not correct.

His schoolmate secretly shows him a Christmas tree (ie. the 25th of December) to help him.

The weak student then blurts out, "Natale!" (Christmas!)

:D

A funny little story from my school days.

I've taken a break from my French texts for two to three days. Back to my "pleasure": English. I've decided to write about a funny random event at school. Perhaps my classmate would kill me if she saw this but I shall take the "risk".

The last sentence needs a second conditional structure: if she saw this.

So, this happened last year, in the history class. Our teacher had decided not to note use teach us anything new that day, but instead just to ask us some questions to make for a revision of the other lessons. In talking about our history, at some point, the teacher asked my classmate how Ţara Românească (Wallachia in English) was founded, when it had taken place and by whom. She gave a good response (Radu Negru), although she was very nervous and had emotions shaken up. But I think the teacher had noticed she was a little insecure and doubtful, and asked her, "But who was this personality, Radu Negru?"

I'm sure she would have known the right answer if she hadn't been so uncertain , but she couldn't answer so I whispered "descălecător" to her. Quickly, She immediately opened her mouth and said very loudly, with confidence in her voice, "Yes, exactly!"

Check the third conditional pattern: she would have known...if she had been. Generally, avoid putting a modal verb in the if clause.

Our teacher was surprised by the odd answer and in the classroom was in complete silence, combined with the curiosity on the other classmates's faces. I suppose our teacher had figured it out what had just has been happened and he asked, "What did you mean by 'exactly'?"

So she, very nervous and with a dangerous sincerity said, "What my classmate had whispered to me."

A couple of style notes... quotations in English are not usually introduced with a colon. We use a comma.

Also, a new quotation represents a new thought and therefore a new paragraph.\

cont'd...