I have heard, from many students, a number of incorrect interpretations of the field TA requirements.
The field rules for ME are listed at http://blogs.cornell.edu/maeassocdirector/mech-eng-msphd-field-rules/. I urge students to bookmark http://blogs.cornell.edu/maeassocdirector in your browser.
The part relevant to field TA requirements is copied below.
Two semesters of teaching experience are required of all Ph.D. students. This requirement is usually fulfilled through Teaching Assistantships (TAs). International students whose native language is not English are required to undergo screening by the International Teaching Assistant Development Program (ITADP) and may be required to take courses in English and pedagogy before assuming TA duties. The College of Engineering requires all teaching assistants to participate in TA Training offered by the College. In exceptional circumstances, students may petition the Director of Graduate Studies (DGS) for a reduction in the required amount of teaching experience.
PhD students on a three-year (or longer) fellowship may satisfy the teaching requirement by serving one semester as a TA and performing an additional qualified teaching activity. A qualified teaching activity shall be approved by the DGS and Special Committee chair, and must involve teaching technical material to a group and involve at least 15 contact hours.
I have heard it said that anyone can get out of the TA requirement. this is incorrect. I have heard that those on fellowship have the requirement waived automatically–this is incorrect; you have to do an addtional qualified teaching activity. I have heard that if your advisor wants you to get out of the requirement that it will be waived. this is incorrect.
I have also had people speak to me informally about waiving this requirement, then had them assume that they were all set. The rules require a formal petition. No one can get the TA requirement waived without a petition.