Are these sentences correct?

Are these sentences correct?

You may come into the classroom now. Check that you are on the list.
You have to sit down leaving two free seats between (you( all)/ each other).
You have two hours to write do the exam, from 2 o'clock until half past 4.
Please, place your ID on the desk.
Remove your mobile phone, any notes, and put them in your school bag.
Silence please. I am going to hand out the exams.
Don’t turn the exam over until I say so. (? Don't start writing until I say so? Don't turn the exam in until I say so?)
You may not leave the classroom until at least 30 minutes after the exam begins.
You may start now.
When you finish the exam, raise your hand and I'll come to collect it.
There are 30 minutes left.
Your time is over. Please stop writing.
Remain in your seat until all the exams have been collected.

*These are correct but the tone is a little strict. Please remain in your seat until all the exams have been collected. Kindly remove your mobile phone, any notes, and put them in your school bad. Please keep quiet; I am going to hand out the exams.