INSTRUCTOR AND STUDENT 'HARASSED' BY ANOTHER ACFT.

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FOLLOWING APPROX 1.3 HRS OF FLT INSTRUCTION; A SIGHTSEEING TOUR WAS INITIATED. HAVING COMPLETED NUMEROUS MANEUVERS IN THE AREA; A DECISION WAS MADE TO OVERFLY CANYON LAKE. AS PIC; I FLEW THE ACFT WWARD OVER THE MAIN BODY OF THE LAKE; AT WHAT I WOULD ESTIMATE TO BE 300-500 FT AGL. ALTHOUGH THERE ARE NUMEROUS PERSONAL RESIDENCES ON THE HILLS OVERLOOKING THE LAKE; ONLY 2 VESSELS (BOATS) WERE ACTUALLY ON THE LAKE. THESE 2 BOATS; AND ALL OTHER PERSONS; VEHICLES; AND STRUCTURES WERE CONSCIENTIOUSLY AVOIDED DURING THE 5-10 MIN FLT. AFTER THIS BRIEF LAKE TOUR; WE RETURNED TO SAN ANTONIO INTL ARPT. THE ENTIRE FLT WAS WITHOUT INCIDENT AND WAS; I BELIEVE; CONDUCTED WITHIN THE PROVISIONS OF APPLICABLE FAR'S. THE ONLY ITEM OF INTEREST OCCURRED AS I FLEW THE ACFT EBOUND OVER THE LAKE JUST BEFORE DEPARTING THE AREA: MY ACFT WAS 'INTERCEPTED' BY ANOTHER SMA (Y). THIS ACFT (Y) MANEUVERED TO MY 4 O'CLOCK HIGH POS - A BLIND SPOT. CONCERNED ABOUT THIS; I MANEUVERED TO PLACE SMA (Y) AT MY 11 O'CLOCK LOW POS (I CLBED AND TURNED L). ALTHOUGH SMA (Y) SEEMED TO ATTEMPT TO EVADE MY EFFORTS INITIALLY (IT TURNED TO FOLLOW ME WITH EVERY TURN I MADE); I WAS ABLE TO FLY MY ACFT SO AS TO KEEP SMA (Y) IN SIGHT AT ALL TIMES; UNTIL FINALLY SMA (Y) TERMINATED HIS ATTEMPT TO REJOIN; AND FLEW N; TURNING AND DSNDING SHARPLY UNTIL A VERY LOW ALT OVER CANYON LAKE.