Louise Hamilton, 40, from Doncaster, was diagnosed with breast cancer on 4 March, and underwent an intensive course of chemotherapy and radiotherapy.
Hamilton's only break in her six-month treatment was the week starting 7 September, and she wanted to use this time to take a family holiday, with her husband Andy, 44, and three children James, 14, Jacob, five and three-year-old Brooke.
Hamilton said: "My children and my husband, Andy, have been a constant rock to me. They inspired me to fight and never be defined by cancer.
"We all craved a family holiday so we took the chance to book a short break as a great way to chill after my gruelling regime.
"My youngest son's school, Richmond Hill Primary Academy, authorised this holiday without a moment's hesitation.
"My eldest son's school however, Ridgewood Academy, said the holiday will not be authorised as it is not classed as an exceptional circumstance."
Hamilton said: "I have gone through an operation, 15 weeks of chemotherapy and scans, but this is not 'exceptional' enough.
"I am absolutely livid.
"It makes me feel that I have to be on my death bed to get a holiday authorised."
The family decided to go on a short holiday to Lincolnshire during Hamilton's break from her cancer treatment in September, despite the school not authorising her son's time off.
Ridgewood School has told her the holiday will be marked as unauthorised, but it is unlikely she will be fined.
A spokesperson for Doncaster Council said they have not been contacted by the school to take action against the Hamiltons.
Hamilton said her treatment has proved to be successful and her cancer has gone into remission.
Ridgewood Academy was not available for comment.