not a teacher
The sentence is fine but its meaning depends on context.
We sometimes use "slip" to mean "move quickly", especially if the movement is also unnoticed.
So "He slipped under the table" is quite natural in the way that we might say: "He slipped out the side door" or "She slipped behind the curtains before they entered the room".
It could also mean that he slipped on something on the floor (water, grease, banana skin) and fell under the table.
Interested in Language