As promised, I'm posting you the 5 sentences to be checked.

1) Thank you very much for helping me with my school search.
2) Your colleague wrote in her blog that the involved students come from the fifh and sixth classes. Actually, they are between sixteen and seventeen years old.
3) New York is rich in elegant restaurants, chich nightspots and fashionable theatres.
4) Manhattan acts as a canvas (what does it mean) for the great builders of the 20th century.
5) In 1664 the British took over (I need a synonym) and renamed it New York. The best seasons to visit New York are spring and autumn.

Canvas means "artist's canvas", the cloth upon which the artist paints the painting.

1) Thank you very much for helping me with my school search.
2) Your colleague wrote in her blog that the STUDENTS INVOLVED CAME from the fifh and sixth classes. Actually, they WERE between sixteen and seventeen years old. (If she wrote instead of writes, this is all in the past tense)
3) New York is rich in elegant restaurants, CHIC nightspots and fashionable theatres.
4) Manhattan acts as a canvas (EXPLAINED ABOVE) for the great builders of the 20th century.
5) In 1664 the British took over (SUCCEEDED THE DUTCH)) and renamed THE CITY New York. The best seasons to visit New York are spring and autumn.

1 and 2 are fine.

3) New York is rich in elegant restaurants, chic nightspots, and fashionable theatres.

4) Manhattan acts as a canvas for the great builders of the 20th century.
See definition #4 here: http://www.answers.com/topic/canvas

5) In 1664, the British took power and renamed it New York. The best seasons to visit New York are spring and autumn.

Here are the corrected sentences:

1) Thank you very much for helping me with my school search.
- This sentence is grammatically correct and does not require any changes.

2) Your colleague wrote in her blog that the involved students come from the fifth and sixth classes. Actually, they are between sixteen and seventeen years old.
- It should be "fifth," not "fifh."
- Instead of saying "the involved students," it would be clearer to say "the students involved."
- Instead of saying "they are between sixteen and seventeen years old," you can say "they are aged sixteen to seventeen."

3) New York is rich in elegant restaurants, chic nightspots, and fashionable theatres.
- This sentence is grammatically correct and does not require any changes.

4) Manhattan acts as a canvas for the great builders of the 20th century.
- The phrase "acts as a canvas" means that Manhattan serves as a blank or open space where the great builders of the 20th century could create and leave their mark.

5) In 1664, the British took over (I need a synonym) and renamed it New York. The best seasons to visit New York are spring and autumn.
- Instead of saying "took over," you can use the synonym "seized" or "captured."