A CFI ALLOWED HIS STUDENT TO DSND INTO VNY CLASS D AIRSPACE WHILE ON THE VOR APCH TO BUR.

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INST TRAINING FLT. HIGH DENSITY TFC AREA. VFR PRACTICE APCH. CONGESTED ATC FREQ. STUDENT DSNDED BELOW MINIMUM ALT FOR PUBLISHED SEGMENT OF VOR APCH TO BUR. USUALLY NOT CRITICAL SIT IN VFR CONDITIONS. IN THIS CASE; DSCNT CAUSED ACFT TO PENETRATE VNY CLASS D SURFACE AREA BY APPROX 400 FT. THEN; HAD STUDENT EXECUTE MISSED APCH DUE TO PREMATURE DSCNT; CALLED MISSED APCH TO SOCAL AND WAS ADVISED THAT WE HAD PENETRATED VNY CLASS D AIRSPACE. WAS TOLD TO SQUAWK 1200 AND EXIT SECTOR WBOUND. NO PHONE NUMBER WAS GIVEN; NOR ANY ACTION PURSUED. HAD STUDENT HOLD OVER FIM FOR APPROX 15 MINS BEFORE CONTACTING APCH FOR ADDITIONAL VFR PRACTICE APCHS. SOCAL ADVISED US AGAIN OF THE IMPORTANCE OF MAINTAINING 3000 FPM DURING THAT SEGMENT OF APCH; DUE TO VNY CLASS D. WE ACKNOWLEDGED AND PROCEEDED WITH ADDITIONAL PRACTICE VFR APCHS. ON MOST PRACTICE VFR APCHS; I ALLOW STUDENTS TO DSND BELOW MINIMUM ALT BEFORE DRAWING THEIR ATTN TO THE DEV. WHEN THEY NOTICE ERROR (LAW OF INTENSITY) IT LEAVES A LASTING IMPACT. MY ERROR WAS IN ALLOWING STUDENT TO DO THIS OVER; AND INTO; VNY CLASS D AIRSPACE. PREVENTION: INCREASED ATTN TO THE ENVIRONMENT WE TRAIN IN.