Competition - teachers only use the best teacher!

 Bava Batra (21a) If we have a teacher who gets on with the children and there is another who can get on better, we do not replace the first by the second, for fear that the second when appointed will become indolent. R. Dimi from Nehardea, however, held that he would exert himself still more if appointed: the jealousy of scribes increaseth wisdom.

Shulchan Aruch (YD 245:18) If there is a second teacher who is better than the first We replace the first teacher with the second.

Chazon Ish (Emuna u Vetachon chapter 3) uses this as an example that Halacha tells us what is ethical not whether something appears unethical

This apparently applies only teaching children. Not to community rabbis or roshei yeshiva or any other profession such as doctor or politician