I'm looking for a school where I can take part-time (evening/weekend)
fashion design classes (e.g., CAD design, sewing, fashion art, etc.)
in Hong Kong.

Dear Static, 

There are two places where you can take such courses. 

First of all, at the The Institute of Textiles and Clothing (ITC).
They offer various part-time courses, some towards a degree or a
diploma, some stand-alone courses.

The Institute of Textiles and Clothing (ITC) - Programme
<http://www.itc.polyu.edu.hk/programme/programme.aspx> 

Another option is the Hong Kong Institution of Textile and Apparel,
which conducts several courses, towards diplomas.

Hong Kong Institution of Textile and Apparel
<http://www.hkita.org/ecourse.htm> 

You might also want to consider taking an online course/distance
learning course, which might be more flexible and would fit your
schedule.

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it. Search terms:
"hong kong" "fashion design" school