On Jan 15, I bought 100 shares of a stock at $11.50 for a total of $1,150.
On Feb 25, I bought 100 more shares of the same stock at $10.50 for a total of $1,050.
On Mar 10, I sold all 200 of these shares at $9.50 for a total of $1,950, a loss of $250.
Is the loss from the 100 shares bought in January considered a deductible tax loss since these shares were bought over 30 days prior to the sale? And therefore only the 100 shares bought in February would be a wash sale?
You are correct, only the loss on the 100 shares purchased in February would be considered a "wash sale".
The wash-sale rule prohibits selling an investment for a loss and replacing it with the same or a "substantially identical" investment 30 days before or after the sale.
You sold all your shares on March 10, so any substantially identical purchase made 30 days before or after that date would create a "wash sale" situation. Your loss on shares purchase in January would not be disallowed.