Beatrice Mary Barth was born in London, England, on 11 May 1877, the first of seven children of Ellen Thompson and her husband, Arthur James Barth, a professor of music. The family emigrated to New Zealand in 1881. Arthur Barth had been organist at St Mary Abbots, London, and in Dunedin was appointed organist to All Saints' Church, then to First Church, and later to Knox Church. He became a well-known local musician.
Beatrice Barth was educated by tutors and later she taught the piano privately. She was 'an avid reader and approached her teaching with a scholarly appreciation of cultural values'. Her father died in 1905 and in order to help her mother support her family, she took over his three rooms in Princes Street. Three of her sisters also began to give music lessons and one of them, Ruby, later recalled that these were 'hard times…no money for extras, or sometimes even for the essentials'.
After a time, music examiners recognised that Beatrice Barth and her sisters, Irene and Ruby, had developed a common teaching style, and suggested they work more closely together in a music school. As a consequence, Beatrice, assisted by Irene, founded the Barth Pianoforte Music School (later known as the Barth School of Music) in 1921. Eighteen months later Ruby joined them. They divided the work according to the stages of the pupils: Ruby taught the beginners, Irene took pupils up to grade eight level and Beatrice took the advanced learners through their diploma courses. Under Beatrice's firm direction the Barth sisters built a thriving music-teaching practice, which had a positive influence on Dunedin's musical community and cultural life for several generations.
The sisters had a room each on the top floor of the Bristol Piano Company building for individual lessons, and they had a classroom where they taught theory to the younger pupils. This was also used for meetings to which speakers such as Frances Ross, principal of Columba College, and the Reverend William Hewitson, from Knox College, were invited. They employed a few of their best pupils to teach theory and some senior pupils took their own rooms in the complex. One of the young theory teachers, Jean Hendry, became a leading school music teacher in Otago.
Beatrice Barth and her sisters took an active part in the Society of Women Musicians of Otago, which was an important support group for professional women music teachers and their pupils. Beatrice was a founding member in 1925 and became its second president in 1928; Irene and Ruby also served as president. All three became life members.
Like her father, Beatrice Barth was active in the administration of the Dunedin Centre of Trinity College of Music, London. He had initiated its establishment in 1896 and become the first secretary. When he died, she was appointed in his place and was voted on to the committee in 1941. Her sister Irene replaced her as secretary in 1939, becoming the third member of the Barth family in succession to hold the position. Examination entries grew from 36 in 1896 to exceed 1,000 in 1946. A large number of examination candidates were from the Barth School of Music. In recognition of Beatrice's long-standing record of over 30 years as secretary, the college in 1925 conferred upon her the Fellowship of Trinity College, London, as an honorary degree.
The Barth School of Music was one of the main music-teaching centres of Dunedin until it closed in 1972, and its influence is still apparent in succeeding generations of music teachers. The school was known for its consistently high standards and academic rigour. Said to have been fun-loving and kind, Beatrice Barth as a teacher was thorough, inspiring and dedicated. However, she was uncompromising and rarely praised success in her pupils for fear that it would make them conceited. She did not marry and died in Dunedin on 14 January 1966.