The image is simply way overexposed and thats why it scored so low, regardless of any other intention, reading the original instructions :
1. A high key photograph is one where the overwhelming majority of the tones in the image are above middle gray (including any shadows) yet still maintains detail even in the brightest areas of the subject.
This is clearly not the case here, all details is lost in the brightest areas,which is 75% of your picture
2. Your challenge is to carefully control your lighting and exposure to capture an interesting photo that meets the above definition of "high key". Remember, "high key" does not mean "washed out" or "blown highlights".
This is the key here, careful control light and exposure. Simply a faster shutter speed, higher f stops or other light reducing features would have helped. The subject matter seems appropriate as many high key portraits were send in. Next time really read up on the subject matter and look at sample photographs, learn by example