Many things affect the results of urinalysis tests. Poppy seeds contain opiates which will cause the test to be a false-positive. So those of you who get randomly tested and have a poppy seed bagel/ muffin/ etc, you're all going to come back with a "positive" for drug abuse.
False-negatives can also be made. Certain soaps and cough syrups/ other medications may affect the results that come back, so at the end of the day the thousands of dollars spent on these tests are a waste of money when the results will be inconclusive and more times than not, wrong.
What's the point in testing students for drug abuse if you might not get 100% correct results?