You can book lessons but you cant really book conditions, while 15-20kts would be ideal the instructor should have a range of gear to cover lessons held outside those wind conditions, first lessons can contain a lot of info and flying trainer kites so the perfect conditions aren't really needed, the second lesson being more water based would be ideal to have good conditions but it is normally up to the instructor to what conditions will best suit you. If you call them up tell them its 15-20kts and you want to go out they will probably just laugh at you and tell you to just book in.