''I don't put things off now.''
Ms Bell said the decision to resign had not been easy, and her announcement had left many staff and pupils in tears yesterday.
Although she loved doing the principal's job, it was ''all-consuming'', and she was finding the weeknight and weekend commitments left her little time for family.
''It's challenging as my boys get a bit older. My sons are 12 and 13 now, and it feels like they need me more than ever.
''I've got that precious window of time with them before they move on and become adults - I want to make the most of that time.''
When she was appointed principal, she told the board she would do the job for five to 10 years.
''This will be my seventh year. I feel like I've done what I came to St Hilda's to do.''
Her heart was still in education and she hoped to continue working in the profession as a teacher.
''It will give me the best of both worlds. I'll be able to teach and still spend more time with family.''
Board of trustees chairwoman Nicola Williams said the board was grateful for the long notice period Ms Bell had given, because it gave time for a considered search for a new principal.
''She's been a tremendous principal,'' Ms Williams said.
''She's left the school in such a good space.''