A Tale of Two Students

Before the beginning of each academic year, Australian Universities start sending out their First Round offers of placement to students. These offers are based on three main criteria:

1. What the student said they wanted to enrol in
2. What a University considers to be a "suitable" candidate for each course.
3. Whether the student meets the prerequisites for a course.

The most widely used method to determine "suitability" of candidates, is the calculation of the Australian Tertiary Admissions Rank (ATAR). A student's results from school (and often an external examination based on Year 12 studies) are used to calculate their ATAR, but their ATAR is not a mark, it is a rank between 0.00 and 99.95 and indicates a student’s position relative to all the students who started high school with them in Year 7. Usually, the average ATAR is about 70.00. A student with an ATAR of 70.00 has been ranked in the top 30% of their Year group.
Some degree courses, like Engineering for example, will have particular prerequisites like a higher level of maths as well as physics and probably chemistry. A student without the required level of maths, might be able to "make it up" in a bridging course for instance. Otherwise, a student without the prerequisites would not be considered "suitable".
No-one thinks the ATAR system is perfect, far from it, but it does give us some way to compare a student from one state studying one set of subjects with students from other states studying the same or different subjects.
In 2015, two students I know, let's call them student A and student B, were offered university places in the first round of offers. Both students had studied English, Physics, Chemistry, and the highest available level of Maths. 
Student A wanted to be a mechanical engineer, and spent her spare time stripping down an old VW beetle and rebuilding it, and she had a "scrapbook" of photos and notes about a bridge that was being constructed on her route to school.
Student B didn't know what she wanted to do, but both her parents were Lawyers so she supposed she'd probably just "do" Law. From what I saw, her main interests in life were makeup, clothes, jewelry and boys, but not necessarily in that order.
Student A did not rank high enough to get offered a place in an Engineering course, so she opted for a Science degree (more maths, physics and chemistry) in the hope that after first year she would be apply to get admission into Engineering. Student A was disappointed, but was hopeful that she would eventually achieve her goal.
Student B did not rank high enough to get into Law, but did rank high enough to get into Engineering, and took up the offer. She wasn't really sure what engineering was, and had spent most of 2014 telling me that she hated physics.
Fast forward to the end of 2015. 
After one full year of study in the Science Degree, Student A has achieved excellent results, and, has decided to continue her studies in maths and physics. Her comment was, "why play around with the results of someone else's work, when you can be there at the cutting edge trying to come to an understanding about the forces that shape our world".
Student B dropped out of her Engineering studies in the middle of 2015. Her comment was, "I've never been so bored and confused in my life. I had no idea what they were all talking about, and, I really couldn't have cared less". She is currently enjoying her backpacking holiday abroad. Her parents are hopeful that when she returns she will have a better idea of what she wants to study.
The moral of the story for Australian senior high school students is, 

1. Choose your school subjects wisely (make sure you cover prerequisites for any University course you think you might be interested in)
2. DO NOT become obsessed by some perceived "need" to achieve a particular ATAR, It is not the only route to getting where you want to be, and you might discover that your interests actually lie somewhere quite different any way.
3. DO NOT think that, because you get a high ATAR you MUST do Medicine or Law. If science is not your thing, medicine probably won't be either. If you are not obsessive about details and structuring an argument, Law is unlikely to excite you.
4. DO talk to your Career's Adviser. They really can help narrow down the options, before it's too late to change.