Physics for self-study while still in school

How should one spend one's spare time if one wants to learn modern physics because what is offered in school is not challenging enough?

Set your goals on something concrete that you really would like to know but do not yet understand at all, such as ''Why does water freeze?'' or ''What is an elementary particle?''. (And if that is settled, go for something more advanced but again concrete, etc.)

Given the goal, search for the answer, starting with Wikipedia (and later Google Scholar), and backtracking on not or only partially understood concepts and topics until you feel firm ground. This is the only way to see what is important and why. It may take you years to fully understand the answers (some questions of this kind are still poorly understood even on the research level), but it will unlock all your intellectual capabilities, and make you an independent thinker.

For example, suppose you somewhere picked up that 'chiral gauge theory' is important in elementary particle physics, but you have no idea what it is about. From en.wikipedia.org/wiki/Chiral_gauge_theory, you discover that you need to know, e.g., the concept of a chiral fermion. This is one step of backtracking. Clicking at the corresponding link, en.wikipedia.org/wiki/Weyl_spinor tells you that you need to understand the concept of an orthogonal group, another backtrack. Clicking again, en.wikipedia.org/wiki/Orthogonal_group reduces it to linear algebra, which you may have started to study already. (If not, you'd backtrack further.) The recipe works like this in every other case, too.

If you are led to the same concept multiple times, you know that this is something important - you should spend more effort on understanding a concept the more frequent you need it, while for something found only once, a more superficial understanding may suffice (for the time being). After a number of such backtracking searches you'll know the basic stuff that needs to be learnt first. After having gotten the basis you repeat the whole procedure, finding the next layer to get familiar with, etc., until you understand everything there is to understand.

If you get stuck somewhere, work on other basic parts; the understanding of one part often complements that of other parts. If you are really stuck in some area for a longer time, ask in one of the forums on the internet. It may then also be time to consult some textbook on the subject that gives you a systematic introduction.

Note that the attitude is more important than the order in which you tackle things. Look at my theoretical physics FAQ, especially at the first few sections of

Chapter C4: How to learn theoretical physics: