For all he knows, the students went to a site with study materials to prep for a test. Generally previous exams are good for checking that you understood a top

For all he knows, the students went to a site with study materials to prep for a test. Generally previous exams are good for checking that you understood a topic but you can not really study a topic only from past multiple choice tests. If they had studied the topic and gained some understanding, they would have noticed that the answers are wrong. Even just looking at two different past exams would probably reveal that something is off.