Just ask them to better describe the problem and encourage them to do the research. Lots of kids have about a month before the end of school and they may not have a clue how to analyse the problem or define the problem. If they then come back with a better description and better defined questions then you can figure they are at least a little serious. Give pointers and hints as to where to look and what they need to look for. (Data sheets have a lot of info, but which info actually means something???) If they don't come back with better questions and a better idea then they were not very serious about learning/accomplishing anything. If they do come back with better questions and ideas then we may potentially have found another addict to string along...
There is a lot of stuff that has become intuitive to us old guys (I'm 54) that the noobies will have no idea about - current limiting resistor on a diode? What's that for?
In some areas it is easy to give help - Add some serial print statements so you can see what that variable is doing. Others require more restraint - Just code it this way...
Some of these kids have never done anything like this before. Heck - they haven't ever taken an appliance apart and tried to re-assemble it to see what would happen. They have never written a computer program (they didn't have a clue you COULD write a computer program...). Much of what they are "taught" in school is to get an answer, not solve a problem. They think the result matters, when we all know that the process is much more important. No one ever learned anything when all they had was success. Failure is much more instructive. They think that everything should be easy and you just throw money at it. They think the iPod and iPhone are magic boxes and have no clue that a simple board like the Arduino has so much more magic hidden inside. Some will fail miserably and some will be addicted. Some will fight through the frustration, and some will quit because they have been taught that everything should come easy.
Be patient with them. Some will give up after the first post and others will do some pretty dumb stuff but learn a lot from the experience. Just like we did. And some of their questions are no worse than some of us geazers who are trying to get the old and lazy brain cells to work again...