ברית מלח עולם היא י"ח, י"ט
It is an eternal, salt like covenant
In Parshas Trumah, The Torah writes in relation to the construction of the Aron ועשית עליו זר זהב סביב – You shall make for it a golden crown.
The Gemara (Yoma 72b) explains that the word זר can be translated as “crown” or as “foreign/strange”. Hence the Possuk reads as follows: “If one merits to study and put into practice the Torah, symbolized by the Aron, the Torah will be his crown, i.e. his Torah knowledge will remain intact. Conversely, if one fails to keep the Torah, it will become strange to him, i.e. he will forget his Torah knowledge”.
The נצי"ב applies this concept to the כהונה being compared to salt in our Possuk. Just as salt enhances the taste of food when added sparingly yet can ruin the taste of food when over-applied, so too a כהן when he performs his duties selflessly - לשמה - is deserving of the crown of כהונה. However a כהן who performs his duties שלא לשמה estranges himself from the crown of כהונה.
As mechanchim we too must measure the messages we give our students. The same message in a small dose can be harmful in a larger one.