Rearrange the parts to make a meaningful sentence.

P. students leave the hall

Q. collected every answer sheet

R. before the invigilator has

S. on no account should

Explanation

1. 01

   "On no account" at the start turns the verb round: S "on no account should", then the subject and a plain verb: P.

2. 02

   R says until when, and "has" needs a past form: Q "collected every answer sheet".

3. 03

   So the order is SPRQ: "On no account should students leave the hall before the invigilator has collected every answer sheet."