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The 60-year-old officially retires today and a farewell function will be held at the Edgar Centre tonight.
Since his diagnosis, Dr Galloway said he had been able to continue working "reasonably successfully" for the past six years.
Last year, he announced he would continue working for as long as his health would allow, but was likely to stand down some time this year.
He said the school had grown considerably over the past decade and he had found it increasingly challenging to keep up with the required workload.
"The school has grown and it's the number of hours that don't fit into what my body can do.
"The time seems right for both the school and me, and I'm excited to be entering a new stage of my life."
Over the past few months, he has been passing over the reins to acting principal Leanne Stanton.
Mrs Stanton will hold the position until the school’s board of trustees appoints a new principal.
Board chairwoman Maree McDonald said Dr Galloway had left the school in excellent shape.
During his tenure, it had grown in diversity and roll size, embarked on significant property projects and established innovative teaching and learning programmes.
His career began in 1980 at Watlington Intermediate, in Timaru.
That was followed by teaching positions in Christchurch, Southland and Otago as well work as an adviser to schools for the Massey University College of Education.
His first principalship was at Weston School, in North Otago, before he moved to George Street Normal School in 2005.
Dr Galloway said he had no plans for his retirement.
"I’ve got nothing planned as yet, apart from enjoying a few months’ break and spending more time with my family, including eight grandchildren.
"There will always be a few education-related projects to keep me occupied but I'm looking forward to the reduction in hours on the job."