(Portland, OR)  --  The Portland Public Schools district says classes will shut down for three days in the event of a teacher strike.  Contract negotiations resumed yesterday between the district and the teachers union in an effort to avoid a strike that's been scheduled to begin February 20th.  PPS officials say the three says will be used for "staff training" as substitute teachers are brought in.  The union says yesterday's talks included a proposal that "is fair for teachers and good for kids."