@syanjali-People on F-1 visa are "exempt individuals".

Exempt Individual

Do not count days for which you are an exempt individual. The term "exempt individual " does not refer to someone exempt from U.S. tax, but to anyone in the following categories who is exempt from counting days of presence in the U.S.:

• An individual temporarily present in the United States as a foreign government-related individual
• A teacher or trainee temporarily present in the United States under a "J " or "Q " visa, who substantially complies with the requirements of the visa
• A student  temporarily present in the United States under an " F" "J, " "M, " or "Q " visa, who substantially complies with the requirements of the visa
• A professional athlete temporarily in the United States to compete in a charitable sports event

Source: http://www.irs.gov/businesses/small/international/article/0,,id=96352,00.html