I think you will have to put off taking your gym class until after you deliver...you dont know what kind of pregnancy you will have...but i would think even if its a perfect pregnancy...your not going to be able to complete the class...
You have to let the teacher know, so they can make accomodations
tell them that you are pregnant and hand them a note from the doctor stating that you cant do everything that a nonpregnant girl can do!
Good Luck
The nurse at my school removed me from PE as soon as i informed them of my pregnancy @ 5weeks , i will be 15weeks tomarrow (: