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The 41-year-old was diagnosed with an aggressive form of breast cancer last March, but she went to great lengths to reassure staff and pupils during a special assembly yesterday that her decision to resign was not connected to her health.
After surgery, 12 rounds of chemotherapy and five weeks of radiation treatment, Ms Bell returned to full-time work last October, and her doctors had since given her a clean bill of health.
During her treatment, she turned the adversity into an inspirational life lesson for her pupils by sharing details of her illness, treatment and emotions, in a bid to give her pupils the tools to deal with breast cancer if they experienced it in later life.
''I guess the experience has probably taught me that you can't assume anything about what the future holds, and you've got to do what you really want to do, when you want to do it,'' she said.
''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.''