I have no idea where you measured to get 10". Normally, 4-5" is medium.
The new Wintecs come with a changable gullet system, so the tree size at the front doesn't matter much as you can swap out gullet plates to change the size.
However, Wintecs do not fit all horses. The shape of the front and the shape of the panels needs to match your horse. Some pictures of your school's saddle on your horse, no pad, lightly girthed, would help us determine if the saddle fits your horse. Take a side shot, a rear shot (stand on a stool if you need to), and a 3/4 front angle showing the WHOLE horse's shoulder (top to bottom). Make sure the horse is standing on level ground with its head up.
If you do not already have a horse, then I would advise against buying a saddle.