Putting the milk in *with* the tea reduces the temperature of the water below the level neccessary for steeping the leaves and releasing the 'tea'-ness into the water. This is also why you don't do that satanic abomination of pouring milk over a teabag in a mug. You're killing the delicate temperature-based chemistry.
For those wot want tannin-flavoured milk, then let them go ahead. But for a proper cuppa, the tea needs to be in near-boiling water *before* flavoured with addative-of-choice.
Obwhich, for the last few months I've been doing IT upgrades in in-house catering outlets. It's amazing how many of them offer me a cup of something almost but not entirely unlike tea. They pass it off as "that's how I like it". But that's utterly irrelevant. You're running a canteen, *YOUR* tastes are irrelevant. You make tea to suit the tastes of your *CUSTOMERS*, ***NOT*** yourself.