Walltop Sentries

• WOTC Official Reminder Textofficial rules
  Deathtouch — Any amount of damage this deals to a creature is enough to destroy it.

• WWizards of the Coast2025-10-02
  Official Ruling
  Walltop Sentries's triggered ability checks if there is a Lesson card in your graveyard when it is put on the stack and again when it goes to resolve. If there is no Lesson card in your graveyard when Walltop Sentries dies, the ability will not be put on the stack at all.

Rulings via Scryfall