Where is the Devil?

Another question from internet skeptic sites. 

If god (sic) cast the devil into hell and hell is a place for eternal punishment from which even god cannot grant a pardon, then why do the preachers tell us the devil is walking up and down the hallways of high schools tempting 
teenagers to do drugs and have premarital sex? A warden who allows his prisoners the freedom to go out and kill and rape would be terminated for incompetence.

Answer

After reading that last sentence, I'm tempted to say something about prison furlough programs but I won't.

The simple answer is because, contrary to popular belief, Satan has not been cast into Hell, yet. He won't be sent to Hell until just before Judgement Day. (Revelation 20:10) Satan was cast to Earth. (Revelation 12:9) So, he could be 
roaming the halls of our high schools.

But, actually, Satan himself probably doesn't mess with too many people, himself. He is too busy accusing people before God. (Job 1) However, he does have a large (but not infinite) number of demons who do much of his tempting 
for him. Also remember, people are tempted when they are already led away by their own desires. (James 1:13-14) Demons may set up a situation, but people sin by their own choice. Satan doesn't need to tempt people. Their own sinful nature does that for him.

Another thing to remember about Hell is that it was not meant for man. It was intended to be a place of punishment only for Satan and his follower angels. (Matthew 25:41) Man only goes there because, by rejecting Christ's gift of Salvation, he chooses to side with the devil.