Blade Instructor — ruling 1

Mentor compares the power of the creature with mentor with that of the target creature at two different times: once as the triggered ability is put onto the stack, and once as the triggered ability resolves. If you wish to raise a creature’s power so its mentor ability can target a bigger creature, the last chance you have to do so is during the beginning of combat step.

Blade Instructor — ruling 2

If the target creature’s power is no longer less than the attacking creature’s power as the ability resolves, mentor doesn’t add a +1/+1 counter. For example, if two 3/3 creatures with mentor attack and both mentor triggers target the same 2/2 creature, the first to resolve puts a +1/+1 counter on it and the second does nothing.

Blade Instructor — ruling 3

If the creature with mentor leaves the battlefield with mentor on the stack, use its power as that creature last existed on the battlefield to determine whether the target creature has less power.

Blade Instructor : règles, fonctionnement et interactions (MTG)

Blade Instructor is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)

How it works : Blade Instructor

Original English text — Wizards of the Coast / Scryfall

#1October 5, 2018· Wizards of the Coast
Mentor compares the power of the creature with mentor with that of the target creature at two different times: once as the triggered ability is put onto the stack, and once as the triggered ability resolves. If you wish to raise a creature’s power so its mentor ability can target a bigger creature, the last chance you have to do so is during the beginning of combat step.
No other card is named in this ruling.
#2October 5, 2018· Wizards of the Coast
If the target creature’s power is no longer less than the attacking creature’s power as the ability resolves, mentor doesn’t add a +1/+1 counter. For example, if two 3/3 creatures with mentor attack and both mentor triggers target the same 2/2 creature, the first to resolve puts a +1/+1 counter on it and the second does nothing.
No other card is named in this ruling.
#3October 5, 2018· Wizards of the Coast
If the creature with mentor leaves the battlefield with mentor on the stack, use its power as that creature last existed on the battlefield to determine whether the target creature has less power.
No other card is named in this ruling.