It can be beneficial to sometimes have workouts that are entirely focusing on technique. Be very focused and imagine that you have skis on your feet even when doing dryland exercises. Have short term goals, with a specific goal for each training session. Endurance training also requires patience. Don't expect to feel progress from one workout to the next.
Progress occurs over time through workouts that can be hard and demanding.
It is, first of all, the very hard sessions that will increase your max VO2. The interval sessions, pace workouts and races are a must for training throughout the whole summer and fall. A junior skier who expects to do well domestically should, in my opinion, do two to three hard sessions a week. However, the number of sessions should, of course, fit your training volume and ambitions. If you are training more than four workouts a week then two of these sessions should involve high intensity.