not a teacher
they weren’t able to use me
This seems to mean that he was not required for any of the filming being done on that day. Although he was already made up, they were not able to use him in the scenes they wanted to shoot.
dropped the arm of the cross
This refers to an arm of the cross, not to his own arm. If you picture the large cross, it has an upright section and two arms. It seems that somebody dropped an arm of the cross (perhaps while they were helping him get it into position) and it fell onto his shoulder, causing him an injury.
Student or Learner