I realize most of this will be my unpopular opinion, but...
I thought the characters in the movie were either VERY thin or non-existent. They were pretty much all very forgettable, and it seemed like none of the kids has any character or personality to them. They tried to overextend themselves, though, by trying to have emotional moments with a lot of different characters (like the one guy who went to save or check on his crush, but when he got to her, she was afraid and killed him, yet he still proclaimed his love for her or whatever). That scene may have had some emotional impact if we had any idea who these characters were, but it was just like that was their one scene.
The "bad guys" in general were a joke to me. The one kid who volunteered seemed neither intimidating or all that dangerous, since he appeared to have no idea how to use the one gun he loved shooting so much. I also don't remember any explanation of why someone would volunteer, other than "he's crazy" or something like that. The former teacher was even worse. Why were the military/government guys even taking orders from him?
At the beginning, it looked like the whole thing was a media frenzy, yet that was never touched on again. Do people even know this is going on when it does? If so, why the hell would any kid show up for school the day of a "class trip"?
I don't know, maybe it was all just me. I just didn't like it. I could keep saying things I didn't like, but I know I am in the minority on that movie.
The one scene I thought was great was the one in the lighthouse with all of the girls. I thought the way they all turned on each other so quickly when just the thought of suspicion came up was really well done.