Why is youth crime apparently decreasing in the USA but apparently increasing in the UK?

I think as you correctly say youth crime is increasing in the UK largely because of the factors mentioned by other answerers and yourself viz breakdown of school discipline due political correctness and the huge levels of immigration from former commonwealth and new EU states in Eastern Europe. But where your question seems wrong to me is your assertion youth crime is "decreasing" in the USA. Where do you get that idea from.

You are perhaps right in suggesting there seem to be no reliable figures covering the USA overall since the millenium, and situation clearly varies very much in different areas of the US where news reporting is so much more concentrated on the home town than on the nation as a whole, whereas in the UK everything is far more directed at the nation as a whole. But all sorts of crime by youths and adults in the USA seem to most to be going up in most places, and certainly in moving around cities like New York, Miami, Seattle, Los Angeles or Chicago one continues to get the impression there are crimes are going on not far away from you and the police just can not or do not wish to get involved.

Another factor is that in the US there are vaste areas where the population is all black in most cities and in some places blacks form the overwhelming majority of the school attenders, whereas in UK races are mixed up far more and there is nowhere like Detroit with an overwhelming majority population of blacks. Yet the horrifying incidents of children getting hold of guns and using them to shoot up and kill numbers of other children in their school apparently to try to correct their low self esteem seem to continue to happen in the USA but not in the UK? But the result creates just a tiny percentage of statistics of youth crime. The large numbers of incidents in the USA that go unreported and where there are no convictions do not get into the statistics.

So no this question is just an example of the old saying polticians put across "lies, damned lies and statistics" to confuse voters, statistics being the most successful in confusing. That seems equally true to me both in US and in UK?

In the US, the deterioration of society by virtue of the fact that generations of latch key children exist has been noted for almost twenty years. Police began community style policing and placed armed officers one to a school in many places. This has allowed some future criminals to be detected and aided, and has also fostered positive relationships between other students and the police that did not exist before. The US is attempting to intervene in youth crime in what they call a "proactive manner".

The UK is a much smaller place and made massive miscaluculation is accepting so many from India, Croatia, Russia, Haiti, and of course Muslim countries as well. Many of these groups brought their own mafiosi and reactive attachment disordered children. Normally, in time, these different cultures would have been assimilated by British culture, but the sheer number has made this near impossible. Fixing wanton immigration would go a long way to diminishing UK crime.

Immigration from countries in the map below Dark blue = most crime and corruption and these are the very places the UK is bringing millions of immigrants from

Figures can tell you anything you want them to. Percentages unless based on the same base number can be very nebulous. Social conditions can be much different in two places when a poll is taken