In a studio with proper lighting equipment, you can be very creative in how you light the subject. In this situation, there was a white background but also the sides of the model had white coreboard to reflect as much light as possible onto the model.
This image is the result. High key normally has a lot of highlights and typically has a white background and low contrast. You can tell there is one large lightbox in front of the model by looking at her eyes. You can see a clear reflection of one light. Remember there is also lighting on the background behind the model, as well as white board to the sides and in this unique case, overhead to give as much reflection of light as possible.