Sorry, I have provided the wrong information!!
I have confirmed with teacher that 普通話老師 will only choose among G2 & G3 students. G1 students have Notice at the beginning of the term (Sept) and G1 student could also enrol through the school if they wanted.
There's a Fan Club for GT-MK in Facebook by Mr. Leigh:-
G.T. (Ellen Yeung) College, Mong Kok Campus
http://www.facebook.com/pages/Mong-Kok/GT-Ellen-Yeung-College-Mong-Kok-Campus/199049031742?ref=pymk