It was announced a long time ago that this is a reboot.
Are you absolutely certain about that? I don't doubt you, its just that I find it hard to believe Fox would be willing to reboot a franchise that they still have other movies in the works for (ie, Wolverine 2, the rumored Deadpool, etc). Its entirely possible, but you'd think the execs would realize that rebooting the franchise with this movie (as opposed to just being a prequel to the existing trilogy) while there are still movies in the pipe for the old franchise...it just doesn't make much sense.
Also, as said, if this were a reboot, you'd better believe we'd see Cyclops, Jean Grey, etc. While its nice to see new characters, even someone who has very little comic knowledge can probably recognize Cyclops, Wolverine, Magneto, etc. Most won't know who the hell Havok, Darwin, or Banshee are. In terms of bringing people in to see your movies, familiar characters can go a long way in terms of making money. If you don't believe me, compare how well (financially-speaking, not critically) a Wolverine would do compared to a movie about...Husk.
It also makes the reports from the set talking about the make-up and costumers trying their best to make this movie's version of Beast being made to look like a younger version of Beast from X3 make no sense.
It is entirely possible this is a reboot, its just that in every instance I've read or heard, its referred to as a prequel to the existing movies. Granted, the two White Queens (the one from this movie and Wolverine), the age thing between Havok and Cyclops, etc, do lean toward it being a reboot, but there's a lot of things leaning toward it just being a regular old prequel (especially the foreshadowing of Erik and Xavier playing chess, set up exactly like in X2).