Realize that a child with a cast on his writing arm is unlikely to have any capacity to write. Set up a plan with the teacher to accommodate this child's needs. Typically, the teacher or aide, if there is one, will be in charge of helping write during the school day and the parents will write any homework.
Get a clip board. If your child has a cast on her non-writing arm, it's still difficult to write. The paper moves away and the casted arm isn't as available to hold the paper in place. Use a clip board to secure the paper and minimize the child's frustration at trying to chase down the paper.
Buy some rubberized shelf liner. See the Resource section for a link. This material was created to protect glass and china in your cabinets, but the key element is that it prevents things from moving around. Provide some to your child to take to school and she can put books, the clip board or any other slipper materials on the shelf liner and have it stay in place. This minimizes the need for her casted arm to move around and attempt to secure any run away materials.
Ask if the teacher can modify the testing process. Tests such as spelling test are often as easily completed orally as they are on paper. See if the teacher can allow this type of testing to minimize the need for your child to write.