I told the yoga instructor that I didn’t know arm stand, she said that I was capable of doing it and should practice at home, against a wall.
What is arm stand Mike? Are you referencing handstand(adho mukha vrksasana) or are you discussing forearm balance (pincha mayurasana)?
If it’s another pose please let me know.
Confused in Seattle
With a headstand base like that in your avatar I can understand your blondeness.
Along with several other poses, Pincha Mayurasana requires the student to have certain actions in the shoulder blades in order to be in the pose safely. Those without the requisite actions run an increased risk for shoulder injury as the humerus is often not in joint as it should be. It is frighteningly common.
Therefore I suggest (and teach) students unearth and practice the preparatory poses and actions to do this pose (and the others) safely. That unearthing usually comes from a skilled teacher. If yours is so adept, great. Get the prereqs. If that person is not…
Yeh, I know the headstand photo is wrong, it was just taken as a funny photo for the bike club.
I want to research the whole thing before I do it, then find a suitable teacher to train me in it.
It’s not “wrong” Mike, just that your base is wider than your shoulders and therefore less supportive for the pose. That, of course, forces more weight to be borne by the cranium.
I have excessively long humerus, perhaps this is contributing to that. Anyway I’ll get the instructor to check it next time I do one.
Back to the topic forearm balance
Back to the topic? Okay. Go practice it at home as instructed. 
How do I determine which instructor should be engaged ?