Alma Evers, who has coached pupils through Oxbridge entrance and taught at top-performing King Edward VI girls' school in Handsworth, Birmingham, was marking for the AQA exam board.
She was told to expect 212 poetry and drama AS scripts but only 35 arrived. Weeks later another 50 were delivered without an accompanying letter or mark sheets.
She asked repeatedly for the sheets and eventually received lists of hundreds of names. "I had 17 sheets to wade through with 238 candidates on them, although the cover sheet said it was 253. A list of 12 centres was included but I had received scripts from just six."
After complaining to the exam board she was sent more incorrect lists. Now she says she is unlikely to mark exam scripts again.
"It is scandalous to think that there is such a lack of professionalism in an organisation with power over the future of thousands of 18-year-olds. No wonder it is hard to recruit examiners."
Ms Evers added: "I followed the marking guidelines as closely as possible but if it had been students' work I was looking at in school, I would have marked very differently."
George Turnbull, AQA spokesman, said: "Teachers have to set aside personal views and apply the same rules for marking.
"It would seem that in terms of administration we have fallen short of our usual high standards. We apologise to Ms Evers and will look into her complaints."
AQA was heavily criticised this summer for estimating the grades of 2,000 A-level pupils because it did not have enough markers to check papers.