The instructor is correct. Most airlines use a "balanced field" concept to determine t/o data. The V speeds (with a few minor adjustments for wind and slope) will remain the same no matter what the runway length is. The gross weight for t/o will be limited on the shorter runways.
There have been a few very complete discussions here on that subject here on PPrune over the last few months. You may want to go back and review them.
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