Can this be done? I plan on homeschooling my 14 year old who is in ninth grade I was told to send a letter now to the district to inform them. If by September I were to change my mind can I then enroll him in any school of my choice or how does it work? I live in NYC

You have a deadline to inform your school district and other records to keep

http://www.hslda.org/laws/analysis/New_York.pdf

1. These home school regulations require parents to do the following:
a. submit a notice of intent to home school to the district superintendent by July 1 (the beginning of the school year) annually, or within fourteen days of starting home schooling during the middle of a school year.
b. subsequently, fill out an Individualized Home Instruction Plan (IHIP) form by August 15th or within four weeks of the receipt of the IHIP form from the school district (whichever is later), containing (1) the child's name, age, and grade level; (2) a list of the syllabi, curriculum materials, textbooks, or a plan of instruction; (3) dates for submission of quarterly reports; and (d) name of the persons giving instruction.

New York NY-2 c. maintain records of attendance (180 days). Only required to be submitted upon request of the superintendent.
d. file quarterly reports giving (1) the number of hours of instruction during quarter, (2) a description of the material covered in each subject, and (3) a grade or narrative evaluation in each subject (the superintendent has no authority to judge the adequacy of these reports); and
e. file an annual assessment with the last quarterly report. The assessment can either be a norm-referenced achievement test, or a written narrative evaluation.
The achievement test can be administered by a certified teacher or by another "qualified person." A certified teacher, a home instruction peer group review panel, or other person can conduct the written narrative evaluation. Unless the assessment is administered at the local public school or a registered nonpublic school the parent is to choose the individual "with the consent of the superintendent." We suggest simply notifying the superintendent of your choice in the third quarterly report.
For grades one through three, the written narrative evaluation may be used. In grades four through eight, the written narrative evaluation may be used every other year. Beginning with ninth grade, standardized testing must be done every year.