Why is the skin considered to be an organ?
Because it contains more types of tissue.
By definition, an organ is a group of at least two different tissues that work together to accomplish similar functions.
I know most of the people think about organs as " oh, the organs are the heart, the liver and the stomach ", but it isn't like that.
The skin contains both epitelial and conjunctive tissue. They protect the body from mecanic shocks, pathogens, some substances ( mostly hidrosoluble ones ) etc.
I guess that makes sense now, right?