Three others transferred to different schools rather than be tested. Although private schools can make drug tests a condition of enrollment without inviting lawsuits, public schools that propose to test their students have raised the prospect of a legal battle. The district attorney of New Orleans has argued that hair testing at schools is constitutional because it meets the criteria for protecting health and safety concerns stemming from drug use. The ACLU plans to challenge drug testing in New Orleans public schools as a violation of protections against unreasonable search and seizure and invasion of privacy in both the U.S. Constitution and the Louisiana constitution. Hair testing is used because when people ingest drugs, the substances travel through their bloodstream and are deposited in hair follicles roughly in proportion to the amount taken in.
Traces remain in the hair, which show how long the drugs have been used.
Since hair grows at the rate of about a half-inch a month, the test uses the inch and a half closest to the scalp to detect drug use for the last 90 days. The Department of Justice lists the following advantages of testing hair rather than urine for illicit drugs. Hair greatly expands the time window for the detection of an illicit drug. Urinalysis of a single specimen generally can detect the presence of drugs for a period of several days to a week or two, depending on the drug. Hair analysis can detect drug use for several months or more, depending on the length of the hair.