You should adapt the length of your sentences to fit the subject youare describing.
Long sentences can be used to slow a description down to create a sense of relaxation or time dragging. Short sentences are more punchy, quick and dynamic, and are good for describing dramatic events or action.
By varying the length of your sentences you will be able to show the examiner that you are thinking carefully about your writing, and that you are consciously creating effects for the reader.
For example: "The English lesson seemed, to Kevin, to be dragging on forever, as Mr…