Joining the TMS is voluntary. But, some Elders make it mandatory. I remember an Elder in our cong that just kept assigning talks to individuals who asked to quit. Rather than argue with him, they just developed a brief sickness the night of their assignment. Yes, many Elder wives have resigned from the school.
You can ask to be not assigned for some time, but, they may or may not try to talk you out of it. They can't do anything to you if you quit. Maybe a special needs part. Don't wory, you won't be the only one sweating in the audience.