When Amanda Haehner was installed as president of the NASUWT union in 2008, colleagues from across south London showed their support by wearing badges of penguins, her favourite animal.

As well as being cute, the fact that they huddle together to protect themselves from storms expressed Ms Haehner’s feelings about the role of the union.

But in case anyone had the idea that she was soft, a colleague who worked alongside Ms Haehner for 25 years recalls another memory of her campaign for the presidency.

Her public slogan was “realism not rhetoric”. But in private Ms Haehner - known as Mandy - took a harder line. “Cut the crap, it’s all bollocks,” she would regularly say about the bureaucracy imposed on teachers.

Ms Haehner, who has died suddenly from septicaemia aged 48, was brought up in Cambridge by parents Ralph, a former German prisoner of war, and Daphne.

Known for her acerbic wit, feisty character and independent mind, she was never shy of engaging in arguments even as a child.

But she also displayed at an early age the commitment to improving conditions for others that came to define her adult life, fundraising for National Children’s Home, now Action for Children, and being a hospital radio volunteer.

She trained as a teacher at Roehampton Institute, then part of the University of London, completing her studies in 1982.

Although she had trained in religious education, Ms Haehner started her career teaching English at Archbishop Lanfranc School in Croydon, south London.

But at the age of just 22, she was diagnosed with cancer. Her battle against non-Hodgkin’s lymphoma took her out of school for 18 months while she underwent an exhausting course of chemotherapy.

Friends, family and colleagues have all paid tribute to the determination she showed in fighting the disease and resuming her career as quickly as possible. Her resolve was tested again when her only brother, Duncan, who was training to be a Savile Row tailor, died suddenly at the age of 31.

Ms Haehner’s colleagues were not always aware of her problems. She also suffered from the fatigue syndrome ME, but was not a person who liked to “make a fuss”.

At work, her involvement with the union grew. She had been an activist since 1983 and then went on to become a national executive member for south London boroughs from 1994.

She chaired the education committee for several years and was elected junior vice-president in 2006.

As she climbed the union hierarchy, Ms Haehner always championed the classroom teacher, reminding colleagues what it was like to be taking a class of Year 10s on a wet Friday afternoon.

She was highly praised for cutting through the endless jargon in education and getting to the nub of any issue. She was also known for her quick ripostes. She was not the type of person who only thought of the sharp comeback half an hour after a conversation had finished.

Ms Haehner moved schools to St Mary’s Catholic High in Croydon in 1990, where she taught English and citizenship.

St Mary’s headteacher Ejiro Ughwujabo paid tribute, saying Ms Haehner’s attention to detail made him better at his job. She never allowed him to be off his guard.

As well as holding management to account, though, she was also keen to break down the barriers between staff, often corralling colleagues into the pub on a Friday after work.

Colleagues remember a dedicated and highly skilled teacher who they were looking forward to welcoming back next year after her full-time union sabbatical.

Proof of the affection and high regard in which Ms Haehner was held came on Wednesday when the school closed so that all staff could attend her funeral at Cambridge crematorium.