I agree with Pump2Duncan, she should be able to show the teacher that she knows the material in a way she can be successful at it. It certainly could be test anxiety. What specifically is tripping her up? If she not lining up the columns to calculate correctly (graph paper could help with that)? Is she overwhelemd by the number of problems on the test (Give her the test in smaller chunks)?
Could it be dyscalculia?
These test tips might help:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism