Students are forbidden to bring their mobile phones into the classroom during the exam.

Teaching assistants usually say that cell phones are not allowed in the classroom or please don't use them during exams.

If you want to write it on the blackboard, just say that cell phones are not allowed.

If you want to broadcast to many classrooms before the exam, it will be more comfortable to say that cell phones are forbidden during the exam. It is ok to ban using mobile phones in the exam, and the tone is blunt.

If you say it verbally, it is enough to say that you are not allowed to use mobile phones.

Look at the situation and find a suitable tone.