The International Science Olympiads is a set of international competitions in which 20-100 countries participate each year (different subjects have different popularity).

Since different schools and countries follow different curriculums and standards, the international science olympiads often serve as a good way for universities to evaluate an international student (if the student has participated in the contest).

There are different contests for each of the following subjects - mathematics, informatics (computers), physics, chemistry, biology, astronomy, earth science, linguistics, and philosophy.

So regardless of what you are interested in, chances are there is a contest or two you are interested in. I encourage you to get started in these contests as early in your school life as possible.

To learn more about the olympiads, you can check out this Wikipedia page: International Science Olympiad. For each olympiad, there also exists a community on Commonlounge. Make the most of them. ( Communities for all International Science Olympiads ).

P.S.: There are a bunch of 'fake' olympiads which are quite popular in India. They do lots of marketing, but don't have the reputation and reach as the official one talked about above.

P.P.S.: Some of the communities might not have much content or discussion going on in them, but feel free to ask or say anything you'd like to and I'll try to respond to as many things as I can.