Why it's not a sequel.
Just saw the movie today, and it's not a sequel. First of all, while Kiefer Sutherland is in both movies, he's playing a different character in each, with different names.
I went into the theater thinking it was a sequel, and because of that, I expected Sutherland's character to somehow be more involved in the plot. Like, "I've been through this before, so here's what you have to do." But there was nothing like that. He was just the students' teacher and provided no link at all to the previous movie.
Secondly, there was NOTHING to link it to the previous movie. They didn't mention this had happened before, nor did then imply it. It had basically the same plot as the first, with changes in characters. That makes it a remake, not a sequel.