(Portland, OR)  --  Half a dozen juveniles are facing charges after allegedly spitting on a security guard while arguing with him at TriMet's Gateway Transit Center.  The Portland Police Bureau says the incident began Tuesday night after the security guard asked two of the juveniles to stop smoking on the TriMet platforms.  The youths allegedly responded by pushing the guard to the ground and then punching him and spitting on him several times.  Police responded to the scene and arrested five boys ages 12 to 17 and a 14-year-old girl.  A seventh boy had previously been reported as missing and was returned to his parents.