I am a foster parent whose current placements' parents are drug users. They passed a few UAs (failed only one) but when a hair strand was done it showed extensive use of several drugs.

I am curious as to how they were able to mask their usage in the UAs, and if it is possible to mask usage in hair strands (dad shaved his head, mom cut her hair short and dyed it red recently)

Not really, they may have stayed clean short times before test.

On shaved head, they go to other body hair, if they need to. Some if they shave all areas may show it as a failed test since they are trying to cheat

I am a foster parent whose current placements' parents are drug users. They passed a few UAs (failed only one) but when a hair strand was done it showed extensive use of several drugs.

I am curious as to how they were able to mask their usage in the UAs, and if it is possible to mask usage in hair strands (dad shaved his head, mom cut her hair short and dyed it red recently)

A positive hair test is almost always an indication of chronic drug use*. Not very effective for detecting low or infrequent use. Also Hair Drug Testing (HDT) is more apt to detect certain types of drugs while missing others.

Cutting hair is effective if the individual hasn't used drugs for over 100 days (or thereabout) and harsh cosmetic treatment (bleaching, dyeing etc) hair greatly reduces amount of drug present.

As Fr_Chuck noted, urine analysis is time sensitive. Most drugs will be undetectable in the range of 2-4 days. So brief abstinence is all that is required.

William J. Closson, PhD, Forensic Toxicology Services (http://www.drclosson.com/drugtestinginformation):

Galanter & Kleber, 2011 Psychotherapy for the Treatment of Substance Abuse, American Psychiatric Publishing, Wash, DC, p. 58 (http://books.google.com/books?id=NEWpRlwUAmcC&pg=PA58&lpg=PA58&dq=marijuana+incorporation+into+hair&source=bl&ots=vEelAPWmux&sig=7I9Z-DIiT0tv_2eg6X2fz7NSeA4&hl=en&ei=5oTiTsTlOIv22AXV5smyBA&sa=X&oi=book_result&ct=result&resnum=9&ved=0CHMQ6AEwCDgU#v=onepage&q=marijuana%20incorporation%20into%20hair&f=false)