Falling Skies/Fridge

Fridge Logic
Are these aliens actively hunting humans or not? The humans sneak around, but their numbers marching down the streets in broad daylight and in full view of the big mothership/building and their busy, loud camps would seem to stick out like a sore thumb. What happens to teenagers? The Skitters capture kids and kill adults, so where is the line drawn with teens? Do they check school I Ds?
 * I don't think they're actively hunting humans, but capturing/trapping what they need for slave labor, and making sure other humans don't congregate in too large of numbers. They mentioned having to break up into ever-smaller groups early on in the first episode for just this reason.
 * The way I understand it, the aliens are effectively much less powerful now than duting the invasion: Back then they relied on nukes, but now that they have landed they won't harm their own kind by using We Have Reserves tactics. If this assumption is correct, the humans need to stay not only far enough from the aliens to not get caught, but also close enough to the aliens to be immune to orbital strikes.
 * Plus, it's a lot harder to go around trying to take out small pockets of resistance than massed armies or pockets of civilization. Also, if they may be pragmatic - if they need/want to harness children, then allowing humans to procreate would be a good thing; while they could capture humans and perhaps try to force or artificially impregnate them, this would require a lot of specialized research and tech they might not have or be willing to commit in the short term.
 * They probably are looking more at biological maturity than actual year-of-birth. Of course, that doesn't answer where the line is drawn.
 * The idea that there is some aspect of choosing kids based on biological maturity (or lack there of) might be supported by the fact that the Skitters keep Karen and release Hal. It has been established that Karen is 17 and Hal is 16. Why keep the older one and use the younger as the messenger? Also in Sanctuary 1 it was mentioned that the Skitters "don't care about the younger ones." Looking at the work groups you don't see kids younger than 11-12, even Eli Russell was probably about 10. This would suggest that they are particularly looking for barely pre-pubescent and pubescent kids, not just anyone under a certain age line.
 * Two things here. One, releasing Hal was probably luck of the draw. They needed one to go back and tell the others, to serve as a warning. Two, and far more important, children under the age of ten would not, and indeed, do not, make effective work crews. Their smaller bodies are more prone to injury, and have less physical strength. They probably do take little kids too, but we just don't see them because there's no need to go to where those kids are housed.
 * "Teenager" is actually a relatively new, 20th century designation brought on by placing the transition to adulthood at the age of 18. Many cultures placed the transition to womanhood/manhood much earlier - as just one example, note the Jewish ritual of Bar/Bat Mitzvah at the age of 13. Everyone develops biologically and mentally at a different rate through adolescence (12-18) some lagging behind in one way while moving ahead faster in another - and 18 is a point when almost everyone in an age bracket has developed. Teenagers are neither adults nor children, but a vast hodge-podge of humans in different states of maturation - so while we may wonder what aliens thought of this phase, they probably looked at a group of teenagers and just said "This one's good, that one is too old..." etc.
 * Note how looking up "teenager" on Wikipedia redirects you straight to "adolescence".

They've discovered that the skitters. Apparently a natural ability. They're using this to monitor the area in case skitters approach, which makes sense. But why haven't they realized yet that they might be able to build a skitter jamming device? Prevent the skitters from communicating with each other tactically, communicating with the mechs or harnessed kids?
 * Perhaps the issue is one of power. They don't have the ability to put out a powerful enough radio signal to effectively jam the Skitters. There is also the fact that the Skitter's likely reaction to such jamming will be to target the source area with everything they have and fire on it until nothing is left.
 * It would be difficult to coordinate such an attack if your method of communication was jammed. And they'd have to triangulate to find the source of the signal which would take time. At least for the first few tries it should be a success. The power might be an issue though.
 * They started to try in the season finale. It's actually a major plot thread.
 * Likely also lack of information; there's a lot of bandwidth in radio waves so unless they can pinpoint wavelengths and such, the power and resources to build something to cover the entire range at a level enough to break communications would be prohibitative.

Wow, JFK High School sure has its share of cages and places to lock people up! There's a ready-made Skitter cage, and a fenced off area with a gate in the boiler room!
 * Would you want to give High School students ready access to (possibly dangerous) secluded places on campus? Some simple gates make sense to keep kids safe and out of trouble. And the skitter cage was obviously thrown together by the resistance, IIRC you even see them welding part of it together at one point.
 * True, but a boiler room is better closed off with a simple locked door.

Fridge Brilliance

 * Gee, these people complain about pulling guard shifts more often than you actually see any guards in action or at least guarding - none of them really care, and they're all burned out from doing this for months. That's why you never see any guards in places or at times when guards are really needed.