BIRMINGHAM, Ala. (AP) — A one-time Alabama Teacher of the Year has submitted a letter of resignation after being informed she is unqualified to teach the fifth grade.

In the letter obtained by Al.com, Ann Marie Corgill says she resigned in confusion about her certification after Birmingham and Alabama Department of Education officials recently said she was not qualified. Corgill says she’d been teaching grades one to six for 21 years. She was the state’s 2014-15 Teacher of the Year.

Corgill says she started this school year at Oliver Elementary School teaching second grade but was moved to a fifth-grade classroom shortly after the semester began.

Alabama’s Department of Education says Corgill has Class A and B certifications to teach primary school through third-grade students.

Birmingham City Schools spokeswoman Chandra Temple said Thursday that the district is working on the matter and has no further comment.