RWY INCURSION. UNAUTH RWY ENTRY.

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THIS WAS OUR 20TH FLT TOGETHER. MY STUDENT HAS APPROX 25- 30 HRS OF INSTRUCTION LOGGED OVER THE PAST 3 MONTHS; ALL OF IT FROM ME; AND I HAVE GIVEN APPROX 1200 HRS OF INSTRUCTION OVER THE PAST 3 YRS. THIS COMBINATION OF EXPERIENCE MAY SOMETIMES LEAD TO A SOMEWHAT LACKADAISICAL MOOD IN THE COCKPIT; AND AN ATMOSPHERE EXISTS THAT ASSUMES THE STUDENT TO BE TRUSTWORTHY OF RADIO CALLS; TAXIING; NAV; AND ENG PRIMING/STARTING WITHOUT THE SAME CONSTANT ATTN GIVEN TO A FIRST-LESSON STUDENT. BEFORE MY STUDENT PRIMED THE ENG FOR START; HE MENTIONED TO ME THAT HE HAD ATTENDED AN AOPA CONVENTION IN ATLANTIC CITY A FEW DAYS BEFORE AND I ASKED HIM ONE OR TWO QUESTIONS ABOUT IT. AFTER LISTENING TO ATIS; MY STUDENT CONTACTED GND CTL FOR TAXI CLRNC BUT DURING THIS TIME; TRUSTING MY STUDENT AND INSTEAD OF LISTENING CLOSELY TO THE TAXI CLRNC; MY MIND HAD WANDERED BACK TO THE AOPA CONVENTION AND SOME OF MY STUDENT'S COMMENTS ON IT. ONE OF THE FEW ITEMS I DID PICK UP FROM THE ATIS WAS THE FACT THAT THE WIND WAS 240 DEGS AT 10 KTS AND VARIABLE. DANBURY ARPT HAS 2 PERPENDICULAR RWYS (17/35 AND 8/26) WHICH INTERSECT AT THE CTR OF THE ARPT. MY FBO IS LOCATED ON THE SW CORNER OF THE ARPT. WE PROCEEDED TO TAXI EBOUND ON TXWY C TOWARDS RWY 26. DURING THE TAXI OUR CONVERSATION CONTINUED. I DID HEAR GND CTL CLR A CESSNA CONQUEST AT THE OTHER END OF THE FIELD TO RWY 26. AS WE APCHED THE INTXN OF RWY 17/35 WITH TXWY C; MY STUDENT BEGAN SLOWING THE WARRIOR; AND A MOMENT LATER SEEMED HESITANT AND UNSURE IN HIS OP OF THE ACFT. REMEMBERING THAT THE WIND WAS FROM 240 DEGS AND THAT ANOTHER ACFT HAD JUST BEEN CLRED TO TAXI TO RWY 26; I ASSUMED MY STUDENT WAS REDUCING SPD IN ORDER TO LOOK BOTH WAYS BEFORE XING RWY 17/35. MY STUDENT CONTINUED STRAIGHT AHEAD; ASKING 'AREN'T WE SUPPOSED TO TURN SOMEWHERE HERE?' I RESPONDED BY ASKING 'AREN'T WE CLRED TO RWY 26?' AND TAKING CTL OF THE AIRPLANE; SLOWING IT FURTHER. MY STUDENT RESPONDED BY SAYING; 'I THOUGHT HE SAID RWY 17.' AT THIS POINT I STOPPED THE WARRIOR AND ASKED THE GND CTLR 'WHICH RWY ARE WE USING?' THE CTLR REPLIED; 'AS YOU READ BACK; RWY 17.' I THEN INFORMED THE CTLR WE WOULD TAXI TO RWY 17. HOWEVER; THOUGH OUR WARRIOR HAD NOT TAXIED ONTO RWY 17; AND THOUGH THERE WAS NO TFC LNDG OR TAKING OFF ON THAT RWY; THE NOSE OF THE ACFT HAD CROSSED THE 'HOLD BARS' BY APPROX 5 FT. I BELIEVE THIS SIT COULD HAVE BEEN AVOIDED; FIRST AND MOST IMPORTANTLY; BY ENFORCING THE SAME 'STERILE COCKPIT' RULE CURRENTLY IN EFFECT AT 135 AND 121 OPS. TAXI HAS THE POTENTIAL OF BEING ONE OF THE MOST HAZARDOUS PHASES OF FLT; ESPECIALLY WHEN APCHING A RWY. I WILL PERSONALLY MAKE THIS A POINT WITH EVERY STUDENT AND ON EVERY FLT FROM THIS POINT FORWARD. I DIDN'T HAVE TO REMIND MYSELF DURING THE LATTER HALF OF THIS TAXI THAT I WAS THE PIC NO MATTER WHAT MY STUDENT DID OR WHAT HIS EXPERIENCE LEVEL. AN INSTRUCTOR SHOULD NEVER ASSUME HIS STUDENT KNOWS WHAT HE IS DOING. ALWAYS EMPLOY VIGILANCE; LISTEN CAREFULLY TO ALL CLRNCS AND NEVER SIMPLY ASSUME. SINCE MY STUDENT WAS TAXIING TOWARDS RWY 26; ANOTHER ACFT HAD JUST BEEN CLRED TO RWY 26 (ALBEIT A TURBINE ACFT AND RWY 26 IS LONGER AND PREFERRED) AND THE WIND WAS FROM 240 DEGS; I HAD SIMPLY ASSUMED THE CTLR HAD CLRED US TO RWY 26; OTHERWISE MY STUDENT WOULD HAVE TURNED L AND CROSSED RWY 26 NEAR OUR FBO; WHICH IS A COMMON PROC AND A PREFERRED TAXI RTE.