Tulip Skirt

I finished my tulip skirt yesterday (my grandma tried to teach me how to sew) and now I really hate undo stitches. Sometimes it't realy unnecassary but my grandma told me so...
It looks like this in black (that's the cut; out of a Burda magazine). Mine is a bit longer (down to the knees) but I'll try to shorten it (hopefully my grandma don't recognise, maybe she will find that I'm grown again...). While flickr through some other Burda magazines I fell in love with a dress.... Istarted searching for a dress for the ball at the end of the school year. I don't know if it's supposed to be 'Prom' like and how overdressed we're expected to appear (how long, pompous and impressing the dresses shall be, or more the short-black ones, if we should dance in them and if, how? formal or modern?) I'll try to adapt my outfit to the other ones but I have to be quick if I want to make it on my own. And have to persuade my grandma to help me/to sew it for me/to accept my decision.