Evans drapes a blanket around his shoulder in the story "Evans Tries an O-Level" to create a disguise and deceive the prison officers. This act is a part of his plan to escape from the prison.

Creating a Disguise:
Evans, who is a highly intelligent and skilled prisoner, has been planning his escape for a long time. He knows that his chances of success would be higher if he can disguise himself and blend in with the prison staff. By draping a blanket around his shoulder, he is able to create the appearance of a prison officer or a senior staff member. This disguise is crucial for his plan to work.

Deceiving the Prison Officers:
Evans understands that the prison officers are familiar with the regular staff members but may not recognize every individual. By wearing the blanket like a cape over his shoulder, he can hide his prison uniform and appear different from other inmates. This helps him avoid suspicion and allows him to move around the prison without drawing too much attention.

Confidence and Authority:
Wearing the blanket also gives Evans an air of confidence and authority. It helps him project the image of a senior staff member who is supposed to be in control. This is important because it allows him to navigate through the prison without arousing suspicion. It also helps him maintain his composure and act confidently while executing his escape plan.

Executing the Escape Plan:
Once Evans is disguised, he uses his knowledge of the prison layout and staff routines to carry out his escape plan. He manages to fool the prison officers and other staff members by acting like one of their own. The blanket serves as a crucial element in his disguise, helping him to successfully execute his plan and make his way towards freedom.

In conclusion, Evans drapes a blanket around his shoulder to create a disguise as a prison officer, deceive the prison officers, project confidence and authority, and ultimately execute his escape plan. This small detail plays a significant role in the overall success of his escape attempt.