If you study in class 11 and want to apply to the CA-CPT coaching class then yes you can do so.
But you can not appear for the Common Proficiency Test unless you have completed your 12th standard.
You can apply for the CPT exam once you have completed your 10th standard but you have to clear your 12th standard to sit for the CPT exams.
Many students though enroll for a CA-CPT Coaching class immediately as they enter 11th standard to better prepare themselves for CPT exam.