Your driving instructor was wrong, and not even in the realm of correct. You keep your foot on the break pedal at a light, not put the parking brake up. In no world is sitting a light in neutral with your foot on the brake any more dangerous than Any. Single. Automatic that does the exact same thing if you were to be rear ended, roll forward. Except in this case, with your foot off the brake, the vehicle with an auto is going to roll slightly faster, because it’s having torque applied.
Edit: oh ya, if you’re putting your handbrake up while at a light, it makes you slower to react to changing circumstances. I’ll give a scenario that I saw 3 times today: You’re a new and inexperienced driver and a emergency vehicle comes from around a corner behind you while you’re sitting at a light, and you’re in the outermost lane which currently has the least traffic in it, and you have to move for it to get past. Which outcome seems more likely, A) the new and inexperienced driver smoothly releases the handbrake, applies the clutch and throttle equally while having slidden the shifter expertly into 1st gear, or 2) they shift into 3rd because its probably not the smoothest gearbox for their first manual and then dump the clutch with no throttle, stall, try to fire it up, remember they need to apply the clutch, start it and then stall into 3rd, then same thing after shifting into 1st but the handbrake is still up?
That’s the same as automatic though. Are you also told you put those in park?