A C172 INSTRUCTOR AND STUDENT PRACTICING LNDG ROUNDOUTS TOUCH THE RWY SEVERAL TIMES ON THE SAME LNDG. ATC CALLED THE ACTION ILLEGAL.

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DURING A TRAINING FLT; I WAS INSTRUCTING THE STUDENT ON PROPER ROUND-OUT HT AND ATTITUDE. WE REQUESTED AND RECEIVED AN OPTION CLRNC WITH NO RESTRS. DURING THE LOW-PASS; WE TOUCHED THE RWY MORE THAN ONCE AND WERE ADVISED BY THE TWR THAT THIS WAS AN ILLEGAL OP. THE CTLR EXPLAINED THAT WE WERE ALLOWED ONLY ONE CONTACT WITH THE RWY AND HAD CONDUCTED 'MULTIPLE TOUCH AND GOES' WITHOUT APPROVAL. SINCE WE WERE CONDUCTING ROUND-OUT HT PRACTICE; WE WERE NECESSARILY CLOSE TO THE RWY AND AT LOW SPD WHICH RESULTED IN INADVERTENT MULTIPLE CONTACTS WITH THE RWY SURFACE. ON A SUBSEQUENT APCH; WE REQUESTED AND WERE GRANTED APPROVAL FOR MULTIPLE TOUCH-AND-GOES. THE CTLR WENT ON TO EXPLAIN THAT THE REASON FOR THE REG WAS THAT IF WE HAD PROGRESSED FAR ENOUGH DOWN THE RWY ALLOWING HIM TO CLR ANOTHER ACFT FOR LNDG BEHIND US AND WE THEN TOUCHED THE RWY AGAIN AFTER THEY LANDED; IT WOULD HAVE RESULTED IN AN OPERROR FOR HIM. I HAVE BEEN A CERTIFICATED PLT FOR OVER 13 YRS AND A CFI AT THIS ARPT FOR THE LAST 3 YRS. I CAN'T REMEMBER BEING TAUGHT THIS PARTICULAR REG AND WAS UNDER THE IMPRESSION AN OPTION CLRNC ALLOWED FOR UNRESTR OPS CONCERNING THE RWY. WHILE I HAVE HEARD OF A CLRNC FOR MULTIPLE TOUCH-AND-GOES; I DIDN'T ANTICIPATE THAT THIS WAS REQUIRED IN THIS SITUATION SINCE THE INTENT WAS NOT TO TOUCH THE RWY AT ALL. FURTHERMORE; I HAVE DONE THIS TYPE OF TRAINING BEFORE WITH THE SAME INADVERTENT CONTACT WITH THE RWY AND RECEIVED NO SUCH ADMONITION FROM THE CTLRS.