| Mandatory drug testing usually occurs either in high schools or medium to large companies. It is also an invasion of one's right against search without reason under the 4th Amendment, but neither of the two institutions mentioned have a habit of respecting liberty.
Drug testing at high schools is typically due to parent cluelessness and hysteria - hardcore drug abusers are probably not going to be in school in the first place, and most teachers know which kids are abusing if it's affecting them in a negative way. Most high school testing occurs in the South and Midwest. Due to ACLU lawsuits, drug testing is usually passed through Constitutional concerns as a requirement for extracurricular activities such as debate, athletics or other afterschool clubs. As for companies, mandatory drug testing is required for various positions for different reasons. Menial jobs in urban areas are usually looking to weed out larcenous addicts - never trust a junky. People in dangerous positions, pilots, drivers and such, are sometimes tested, even though impairment tests are a better way of insuring public safety. The other and probably most common reason for mandatory testing is money. Some insurance companies offer rebates to employers that promise Drug-Free workplaces, requiring mandatory drug testing for the employees. Of course, employee morale goes to hell, but that small decrease in cost is worth it to some companies. After all, if your e-mail is being read , your webaccess is logged and your movements are recorded on camera, taking urine, blood and hair isn't that far off. | 
