Word or phrase to describe slacking because you think you already know everything
Solution 1:
You could say the student is suffering from the Dunning-Kruger effect:
The Dunning–Kruger effect is a cognitive bias in which relatively unskilled individuals suffer from illusory superiority, mistakenly assessing their ability to be much higher than it really is. Dunning and Kruger attributed this bias to a metacognitive inability of the unskilled to recognize their own ineptitude and evaluate their own ability accurately.
See also:
- Delusions of grandeur
- Hubris
- Narcissism
- Optimism bias
- Overconfidence effect
- Self-serving bias
- Superiority complex
Solution 2:
The student in question is a victim of his own complacency.
From dictionary.cambridge.org:
complacency noun also complacence, disapproving
a feeling of calm satisfaction with your own abilities or situation that prevents you from trying harder:
What annoys me about these girls is their complacency - they seem to have no desire to expand their horizons.
There's no room for complacency if we want to stay in this competition!
From oxforddictionaries.com:
complacency (also complacence) NOUN
A feeling of smug or uncritical satisfaction with oneself or one’s achievements:
the figures are better, but there are no grounds for complacency