First, I must say that I like this drawing. The theme, though popular, is not generic - Especially this reference picture.
Your technique is also very good. I like the use of dark shades contrasting the lighter ones, which adds to the vision.
My favourite part is Harry's face which in my opinion is the best drawn - Especially the spectacles.
Also, I like the details in the sword, and the lack of details in the background: It helps concentrating on the characters.
I think, however, that in some places you used too sticking out black lines: It can be mostly seen in Ron's collar, but also in some other parts. Also, Hermione's hand looks less realistic than the rest of the drawing, probably in part because of the dark contour.
Overall - Good job!