C172 TRAINEE AND UNKNOWN BEECH ACFT HAD NMAC OVER PRC ARPT.

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OUR ACFT; A C172; WAS SET UP FOR A STRAIGHT-IN APCH FOR RWY 21L AT PRC. PRC TWR TOLD US TO MAINTAIN 6500 FT (500 FT ABOVE TFC PATTERN ALT); OVER-FLY THE RWY; AND RPT ABOVE THE THRESHOLD. WE COMPLIED. AFTER RPTING THE THRESHOLD; WE WERE TOLD TO EXECUTE A TURN TO THE L DOWNWIND AND DSND TO TFC PATTERN ALT. TFC RPTED TO US WAS A TWIN BEECH TURNING DOWNWIND FROM XWIND. DURING OUR TURNING DSCNT FROM ABOVE THE RWY TO THE DOWNWIND; I NOTICED A SHADOW ON THE GND THAT WAS NOT OURS. THE INSTRUCTOR IN THE ACFT LOOKED BACK FOR THE BEECH AND WE DETERMINED THE SHADOW WAS NEITHER OURS NOR THAT OF THE BEECH. ONCE ESTABLISHED ON DOWNWIND; I NOTICED THAT AN UNIDENTED ACFT APPROX 20 FT TO OUR L AND 200 FT BELOW WAS CLBING INTO OUR FLT PATH FROM L TO R. PRC TWR CLRED US TO LAND AND ADVISED US OF AN ACFT DEPARTING THE L DOWNWIND. UPON REALIZING THAT STANDARD EVASIVE MANEUVERS TO THE R WOULD PROVE FATAL; WE IMMEDIATELY TURNED SHARPLY TO THE L TO OVERFLY THE RWY AGAIN. LEADING TO THIS WERE MANY FACTORS. THE UNIDENTED BEECH DID NOT DEPART USING A STANDARD DOWNWIND DEP AS SUGGESTED BY THE AIM. IF HE HAD; HE WOULD NOT HAVE BEEN A FACTOR. ADDITIONALLY; THE TWR FAILED TO PROPERLY SEQUENCE THE ACFT. A CESSNA WAS TOLD TO ENTER THE DOWNWIND IN FRONT OF A TWIN ENG ACFT AND ON TOP OF AN ACFT CLBING THROUGH TFC PATTERN ALT; A SIT THAT WOULD HAVE BEEN NO FACTOR HAD WE FLOWN A NORMAL SIZED PATTERN.