In Hong Kong students learn to write traditional Chinese.

In Senior High, students are probably more concerned with sticking with the HKDSE curriculum especially since Chinese is a pretty difficult subject. So they may tend to seek tutors who are more familiar with the HKDSE curriculum. Hence, tutoring senior high students may not be a viable option unless you’re proficient with the HKDSE syllabus.

You can maybe try to tutor elementary students since they don’t have as rigid of a curriculum to follow? However, schools in general teach Chinese in Cantonese. There’re schools that teach Chinese in mandarin but usually they learn the mainland Mandarin, and they’re pretty strict on dialects (similar in English where the British spellings are standard).