This ability cares about the actual power of the creature in play and not necessarily the printed values on the card. For example, if you control an Honor of the Pure and had a Armored Warhorse enter play, the Mentor’s ability will not trigger. It does not matter that the Warhorse has a printed power of two, it enters the battlefield with a power of three. This is also true when creatures enter play with +1/+1 counters on them raising their power over two. For example, if you have a Dearly Departed in your graveyard a Woodland Sleuth entering play will not trigger this ability.
Somewhat similarly, you cannot have a creature with a power of three enter play and lower its power by one in order to cause the Mentor’s ability to retroactively trigger. For example, lowering the power of a Slayer of the Wicked by targeting it with a Virulent Wound will not cause the Mentor’s ability to trigger.
Just as there are many effects that can prevent the Mentor’s ability from triggering, there are a few that that can help it too. If you are enchanted by Curse of Death’s Hold a Pitchburn Devils will enter play as a 2/2 creature and trigger this ability. In fact, a Kessig Wolf would enter the battlefield as a 2/0 under this circumstance. It won’t remain in play for long, but it entering play with a power of two will cause the Mentor’s ability to trigger.