It doesn't matter whether you 'think' she is flirting, or she actually 'is' flirting. It doesn't matter that you are going to be 21...that makes no difference. This is a teacher/student relationship...which means she teaches you,and nothing else. Its totally wrong to go beyond that in any way whatsoever. If it were to continue/go further,she would be up for disciplinary action,and would most likely lose her job,and never be able to teach again. Do you want to see that happen?