There are some schools that go all year long. They typically go to school for 6 weeks and then have 2 weeks off, all year. That would mean no 3 month summer vacation; instead, they get those 2 week breaks every 6 weeks...they still get the vacation, just not all at once. It actually makes more sense for them to do it this way, since it mimics the real work world better than a 3 month vacation, so I'm not sure why so many people are against it (other than the students!!).
Some schools have no summer vacations because they want to review the overall academic progress that they are making so far. They also do away with summer vacations so as to get their students out of trouble during the summer vacations. This move is yet to be fully implemented by all schools and is still a subject of debate.