Read Maty Ezraty's repsonse:
This is a very honorable question. I applaud your courage in asking it.
Yes, it is possible to become a yoga teacher if there are advanced poses that you are not able to do, providing that you stick to teaching what you do know. There are so many poses in the yoga vocabulary to choose from that it should not be difficult to create class sequences with the poses you are comfortable doing. Most students are only working on basic poses in class. If you stick to the poses that you know and teach from your heart, then there is no reason why you cannot be a yoga teacher.
It is not an absolute that an advanced practitioner will be a good teacher. Some advanced practitioners lack patience and understanding, because they never had difficulty in doing the poses. They never had to struggle, probe, and experiment. Conversely, those who are stiff can become great teachers, because they have had to work harder and dig deeper to be able to do the poses.
Some teachers who have practiced for many years may still be stiff, but due to their years of practice, they are able to teach more flexible students poses that they themselves cannot do. Keep in mind that I am describing teachers who have many years of experience. When you are a new teacher, you need to teach poses that you know and can do. With experience and maturity, you may be able to teach poses that you cannot fully execute yourself.
Now let's look at your example of coming up into Handstand or Headstand. I would not categorize these as advanced poses. If you can do the actual Headstand or Handstand and you understand the correct alignment and the risks involved in doing these poses, but it is the coming up that challenges you, it is quite possible that you can still teach these poses.
Now let's look at another example, such as Janu Sirsasana (Head-to-Knee Forward Bend). Say I am stiff in my hamstrings. That does not preclude me from teaching this pose to someone who can easily put their forehead on their shin. I understand the basic alignment points and the risks of Janu Sirsasana, but I just have stiff hamstrings. This stiffness should not inhibit me from teaching the pose. I have been to classes where teachers tell their students that they cannot do a pose and proceed to use a student to demonstrate how to do it properly.
Teaching is an art that requires much more than the physical capacity to perform asanas. If we think of yoga as just asanas, then we are just exercise teachers. Yoga deals with the mind, the emotions, and the whole being. Asanas are simply a vehicle. It is a mature person who understands the deeper road of yoga. That maturity is a sign of one worthy to teach. If you teach with honor and a higher purpose, then you will know intuitively when you can teach a pose.
Maty Ezraty is co-creator of the first two Yoga Works yoga studios in Santa Monica, California. A former YJ Asana columnist, she travels around the world leading teacher trainings, workshops, and yoga retreats.