Dress code for a young woman visiting and working in China?

I am planning on visiting China and working in a school. I am a 23 year old female and would like to know what clothing items are appropriate to wear in the school and in my free time? Is it okay to wear a bikini at the beach or is it best to wear a swimming costume?
I would also like to know about tops! Is it appropriate to wear tops with thin straps? Or would it be best to wear tops that cover the shoulders?