INSTRUCTOR WITH SPI UNAWARE OF TEMPORARY TWR UNTIL IN LNDG FLARE. NO CONTACT.

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ON THE RETURN TO SJC FROM NEW JERUSALEM I DECIDED TO HAVE MY STUDENT MAKE A SLIP TO A LNDG AT TCY. I WATCHED 1 ACFT IN THE PATTERN AND PROCEEDED TO ENTER THE 45 BEHIND AND WELL CLR OF HIM. I MADE A CALL ON 122.8 ON THE 45 ENTRY AND ON BASE. I WAS FULLY OCCUPIED WITH MY STUDENT AS HE TRIED TO SLIP FIRST TO THE L AND THEN TO THE R. ALTHOUGH NOTHING UNSAFE HAPPENED; WE ENDED UP IN THE FLARE OFF TO THE R OF THE RWY ALMOST HALFWAY BTWN THE TXWY AND THE RWY FROM A VERY ZIGZAG APCH. IN THE FLARE; I MOVED THE ACFT BACK TO THE CTRLINE AND WE LANDED ON THE CTRLINE. DURING THE FLARE; AND WHILE MOVING BACK TO THE CTRLINE; THE TEMPORARY TWR CALLED; 'ACFT ON SHORT FINAL CHANGE TO 12(?).' SURE DID SURPRISE ME TO FIND A TWR THERE! I CHANGED TO WHAT I THOUGHT WAS THE NUMBER; RESPONDED TO THE CALL; ONLY TO FIND OUT LATER IT WAS 121.20. WE CONTINUED WITH OUR TOUCH AND GO AND DEPARTED FOR SJC. AS WE CLBED OUT TO 3500 FT I HEARD THEM MAKE ANOTHER CALL; 'ALL ACFT IN THE AREA; TCY TWR IS IN OPERATION ON 121.20.' THAT ANNOUNCEMENT SEEMS TO ME TO INDICATE THEY CONTINUALLY ARE SURPRISING PLTS WHO TRAIN OVER IN THAT AREA AND ARE NOT AWARE TCY HAS A TEMPORARY TWR. I WOULD HAVE APPRECIATED THAT ANNOUNCEMENT AS I APCHED TCY. I SEE IN THE RIU NOTAMS THAT TCY HAS A TWR IN OP. I SHOULD HAVE CHKED MY NOTAMS TO PREVENT THAT SURPRISE. I REMEMBER ABOUT A YR AGO THEY DID THE SAME THING. I WAS SURPRISED; BUT NOT LIKE THE OTHER DAY; AND STOPPED AND ASKED WHAT WAS GOING ON. AS I RECALL; THEY WERE ARMY RESERVISTS IN TRAINING. IS THAT AN FAA TWR? AM I IN VIOLATION OF SOME FAR? I DON'T WANT TO BE. IT SCARES ME. ANYWAY IT SEEMS TO ME IF THEY USED 122.8 AS WELL AS 121.20 AS THE TWR FREQ FOR THE FIRST CALL; IT WOULD ELIMINATE A LOT OF CONFUSION AND SURPRISES. DURING THAT FIRST CALL THEY COULD INSTRUCT THOSE OF US WHO DON'T GET THE WORD TO CHANGE TO 121.20. THERE ARE ALWAYS 10 PERCENT WHO DON'T GET THE WORD -- IN EVERY SIT. I HATE TO BE AMONG THE 10 PERCENT; BUT I GUESS I WAS IN THIS CASE.