My advice to high school students aiming for a professional work

As a student in high school / secondary school; many have ambitions for a professional job. I found it difficult knowing what to study and what would eventually make one a high earner. My answer is a lot more complicated than just "getting good grades". My advice is from a science/engineering perspective. 

The Situation 

Getting good grade is important in the UK; employers do value this. Unfortunately this creates a barrier to entry. It actually doesn't matter too much what the subject is; so long as minimum grades are met. These can be found on employer graduate hiring webpages e.g. 200 UCAS points across 2 or 3 ‘A’ Levels. Therefore, one can choose a subject in your school with good teachers for an easier course (such as geography) over a more difficult one (such as physics). Personally I took physics for A-Level; but with a teacher being out for 7 months of the course; I suffered a bad grade which affected the university I went to and subsequent graduate employer. So my advice is to choose your subjects based on reliability (i.e., check the past record of the teacher/subject) and the relative ease of the subject. That said, I would say that mathematics is key to science, finance and engineering degrees and industries and should be chosen. 

Internships 

Since it is difficult as a student to know the actual types of work that exist in the workplace; the great thing you can do is use the internet to your advantage for researching industries, the latest news and job specifications. Internships also provide real work experience that would be hard to attain. It will also introduce you to competitive human behaviours you may not have experience being in a sheltered educational life (where money and power isn't as involved). 

Recommended Research  

1. Job sites (such as glassdoor.com) or forums for reading job specs and reviewing companies.
2. Graduate job sites of large employers to review their entry requirements.
3. Google news on latest industry developments for industries you are interested in.
4. Robert Green and David F. D'Alessandro books for understanding the competitive human and organisational behaviours.

Caveat 

My advice is from someone who was in secondary school at the start of the century. Times have changed and employers are offering school leavers work without needing to go through university. I would consider giving this thought; as university is much more expensive now. I would err on the side of not going to university if real work experience can provide the opportunity and wealth that you're seeking (without going into debt); but you need to check employers will accept this. I do know people who've never gone to university (or not well named universities) but took them 5-10 years through work experience to work at top hedge funds.