I think...the reason why yr teacher do that because your spine is not fully stretching...
From my guess..I think you bend your back too much...pls check out this katoo;
http://www.pantip.com/cafe/lumpini/topic/L5289406/L5289406.html#26
See the picture in Reply number 14..I guess yr back is bending like that..that why teacher try to streching yr back..not let it bend...u should suck more your stomach and think more about lengthen your back not collaps back krab:)
Question 2...
It's called "Chaturanga Dandasana".
I do not think it will make your arm bigger...In my opinion..it should make yr arm FIRM...
BUT..big BUT..do not use too much muscular energy krab..check it out from my last katoo...
It so hard to make arm muscle bigger..
even a man who want to have a big arm..need to train so hard..to get it..
Big arm won't come so easily..so do not worry
I do not think it will make yr arm bigger:)
Hope my answer can help na krab.