I think the organization that you joined would give well arrangement. Since Hong Kong also have more than 100 secondary schools which are using English as teaching language. So beside Chinese and Chinese history class, most of the classes are using english. So at least no worry about fall behind. However, that's also true that don't expect every teacher who teach their subjects in English mean they can speak well english. I worked at schools(both very good english school and very bad schools as well). I can tell that don't expect too much. However, at least all schools now with Native English Teachers, so you won't be alone there. No worry about that. And I sure your organisation won't place you at a band5(old banding system) school. Or you will think you are in a gangster movie.
I sure that your hosting family would be friendly and helpful, so don't worry about pick up Cantonese. This dialect is not easy to pick up but not hopeless too. At least you are a student and I think noone would just teach you foul language. Most of my adult students think funny as once they learn Cantonese, the people around them are only teach them Cantonese foul language. I am so ....to explain those stuff.
I don't think you would take any extra Cantonese class because you would have good environment for Cantonese learning already. But still I don't mind to help even that's not a formal class. If you would like to know more about school in Hong Kong and some daily life here. Welcome to send me e-mail to [email protected]
One more thing, we quite get used to see foreigners around us. So not really crazy much even see you are in blue eyes and blond hair. Just if they said you have 'doll' look, don't get mad as it is compliement, don't mean you look girly.
Enjoy the time when you are in Hong Kong. When I see two Swedish are crying in Ikea, I would know who are they now. Sorry, not intend to make fun of you guys. Hope you don't mind. hehehe