Not bad, interesting premise and good writing and pacing. Doses of unbelievable elements grounded with some realistic responses mixed in with obligatory breathless teen romance. 
 But while it is an entertaining read, somehow it is still missing a certain something that would have engaged my mind and emotions 100%. It neither tipped into the intellectually stimulating nor did it make me check my brain at the door for shameless squeeing. 
 I suppose I could rant about how silly it is to go about training elementary school kids to be your final assassination force for the remainder of human kind when you've got so much whiz-bang explosive alien tech at your disposal (including one that happens to map the locations of all the remaining humans). And at one point I kind of had to ponder the probabilities of a random gorgeous alien-infested hottie assassin falling in love with a human just because he stalked her for a couple weeks through a rifle scope. 
 But that would require me to care, lol. So I would not go out of my way to read any sequels.