As with all parts of an essay your introductions should follow a particular pattern. You need to follow a pattern that you are comfortable with and it really shouldn't change from essay to essay. The following is a basic formula I use for writing an introduction to such a question (but that's not say it is the only one)

1st sentence: A general statement realsted to the question at hand
2nd sentence: Illustrate the opposing idea
3rd sentence: Contrast this with your opinion

So lets put this into practice by looking at an original question from the exam:

"Parents make the best teachers"

Well this question is obviously asking who is the better teacher for children; their parents or their teachers, so I am going to begin my introduction discussing a childs education in the first sentence and then follow the pattern through to the last sentence. So my introduction may look like this:

"Over the last 30 years or so, children have been attending school at a much younger age due to changes in the education system and social pressures that dictate that both the man and wife have to work. It is due to this that many people believe that teachers are in fact the best educators for our children. However, I totally disagree with this and believe that parents are without doubt the best teachers for a number of reasons".