By drug testing at an early age we can give most kids a chance to get away from drugs until they are old enough to form their own decision on the matter. It also allows for us to figure out what children are in families that may not be the best choice to raise them and for actions to be taken accordingly.
It should not be allowed since it is very ineffective, counterproductive, expensive, and very invasive to the students. Also since they only do it to the extracurricular activities that are competitive (sports or athletes) it is generalizing that all sport stars do drugs like marijuana, cocaine, and steroids; even though other kids that don't play sports can be doing drugs and people might not know that.
1. Not all drugs are tested for. Alcohol is unlikely to be tested for. There are also constantly new forms of alternative marijuana being produced that can't be tested for and which are legal (at least until lawmakers get around to banning them but even after that they are unlikely to show up on tests), unlike marijuana these alternatives can be highly dangerous and even lead to fatal overdoses. And remember bath salts and the insanity that caused? That was completely legal at first, because it was newly manufactured.
I could go on, the point being drug testing does NOT and will never cover all drugs and oftentimes these drugs that can't be tested for are far more dangerous than the ones they can.
2. Testing kids for drugs does not teach kids anything about drugs. It does however teach kids that adults expect them to like drugs and want drugs. This would make them more willing to experiment when they graduate from high school.
3. This could increase drop out rates from kids who want to do drugs.
Drug testing creates a myth out of drug use and in some strange way promotes drug use. The children need to be informed about the effects and after effects of the drugs in question, in an open debate or even a class. The drugs should not be condemned from the start but an explanation has to be given to its chemical working and consequences of use. The drug testing is the cure that comes later, we should be thinking of preventative approaches.