Can you please check these sentences?

I find it difficult to rephrase to sentences. Thank you.

1) thank you very much for helping me with my school search.
2) In the blog I noticed that your colleague wrote that the involved students come from the fifh and sixth classes.
3) Actually they are between sixteen and seventeen years old.
4) New York is rich in elegant restaurants, chich nightspots and cutting-edge (synonym) theatres.
5) Manhattan acts as a canvas for the great builders of the 20th century.
(I need to rephrase this)
The Dutch first settled on Manhattan in the 1620s.
6) 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 when temperatures reach 21°C.

Make sure every sentence starts with a capital letter and ends with a period (or whatever is correct).

Put commas in the right places.
http://grammar.ccc.commnet.edu/grammar/commas.htm

Check all words for correct spelling.

Use this for finding synonyms:
http://www.thesaurus.com

Re-post the sentences you have real concerns about -- no more than 4 or 5.

Certainly! Here are the suggested rephrases for the sentences you provided:

1) I appreciate your assistance in helping me with my school search.
2) I observed in the blog that your colleague mentioned that the participating students are from the fifth and sixth grades.
3) In actuality, their age ranges from sixteen to seventeen years old.
4) New York boasts an abundance of upscale restaurants, trendy nightlife spots, and avant-garde theaters.
5) Manhattan serves as a canvas for the visionary architects of the 20th century.
(Rephrase: Manhattan is a blank slate for the innovative architects of the 20th century.)
6) In 1664, the British assumed control and renamed it New York.
(Rephrase: In 1664, the British took charge and gave it a new name, New York.)
7) The ideal times to visit New York are during the spring and autumn seasons when temperatures peak at 21°C.