It really depends on what type of gains from the exchange you are looking for. I went on exchange in year 10, and after I finished school and will probably go in uni next year or the year after (damn double degrees!).
High school exchange is best done for a whole year in year 10. This means that you experience a whole year in your host country, really learn a language, get to know your host family and friends so much better. The special thing about high school exchange (which you can also do as a gap year) is that you are completely immersed in the culture, living with a host family and forced to attend school to a certain degree, which is hard to achieve later on. You live with parents who usually are willing to spend time helping you out and will have siblings and probably grandparents and other family members which I guess makes it a more comforting experience for a high school student.
With gap exchanges, I guess there are few types.
1. The gap year in England
You get to look after a whole lot of snotty nosed private school kids, doing their washing, organising their sport, living with other gappies. You get holidays off and sometimes the use of the school gap car. It means that you're earning money (in pounds, which gives you a pretty awesome exchange rate) and are pretty centrally located in Europe, which means it's easy to travel around there. But I don't really see it as an exchange to be honest, because I don't see England's culture as very different to ours, nor is there the language aspect. I see it as a working holiday.
2. The highschool exchange
Much like what I said above, except now, you don't have to care at all about your grades because no one will be looking at them ever. Not the best, because you still have a lot of restrictions, in regards to relationships, drinking, drugs, school attendance and you still have to comply with your family's rules or you will be sent home, even though you are 18.
3. The 'humanitarian' trip
Where you pay to volunteer in a third-world country. The fact that you are going with an organisation makes the parents happy, and you are happy because you are making the world a better place... or are you? Sure you are making a short-term difference whatever you do, whether it is building a school, or teaching English, but in the long term, it isn't sustainable - can and should a country rely on privileged westerners for their infrastructure and education, coming in periods of 1-6 months at a time? I'm not saying it's all bad, just choose your country and program carefully and be realistic about how you are actually going to positively affect the area you live in. The shorter the length of time you stay, the less helpful you will be.
World Volunteer Web:Volunteer tourism & the gap year
4. The do-it-yourself
This is what I advocate, but it is probably the hardest to actually do. It can be hard to organise everything from Australia, and visas can be difficult to get. But it means that you can be flexible and free while you are away and means that you have to rely on yourself a lot more, which I think is important on a gap year, it is a time where you are supposed to be growing up, a rite of passage. I decided to base myself in Denmark for about 1½ years after school finished, and combined working, studying and travelling to have an awesome time. I wanted to learn the language and immerse myself in the culture and I definitely did. I'm proud that I managed to pull it all off, and despite times where I would be thinking fuck, where am I going to sleep tonight, I worked it out. It has been a really great thing to talk about in interviews, emphasising that I worked everything out for myself, instead of just taking a pre-packaged option like most other people.
Uni exchange
Uni exchange is pretty awesome - you can pretty much choose how you want it to be. If you want to learn a language or improve your language skills then you can choose to mix with the natives, or you can have heaps of fun with all the other exchange students. It varies heaps depending on where you go, in terms of the courses you can do, how you find your housing and the support you can get from your uni. You can choose from a year or semester, depending on how long your degree is and how long you are prepared to spend away. Ususally people leave as early as possible and come back as late as possible to allow other time for travel.
If you want specifics, ask someone who has been on uni exchange in the country you want to go to.
I reccomend this for the type of person who doesnt want to go straight to uni, or doesn't particularly enjoy studying - I loved it!