Word or phrase to describe slacking because you think you already know everything

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

A familiar expression is

[He's] resting on his laurels.

From Phrase Finder:

Rest on one's laurels

Meaning

To be satisfied with one's past success and to consider further effort unnecessary.

Origins

The origins of the phrase lie in ancient Greece, where laurel wreaths were symbols of victory and status.

"he thinks he knows everything and therefore slacks" suggests that he is overconfident and overconfidence can negatively affect performance.

• The Weakness Of Overconfidence - We’ve all experienced it before. You take a test that you think is ridiculously easy,and end up getting a B. You get into a relationship and start taking your significant other for granted, resulting in a breakup. You invest in a stock thinking there’s no way it’ll go down, and lose a big chunk of your money

• Well, let’s first take a look at why we get overconfident. Generally, if something requires the full extent of our abilities, we find it challenging and need to use all our resources to overcome the problem. That’s a really hard thing to do, and we generally feel not too confident about it. When things are at about our ability level but don’t strain it too much, we find it interesting and maybe even a bit relaxing. In this case, we can feel confident about our ability to accomplish the task, but know that we still have to work decently hard. However, when the task is extremely easy or below our abilities, we may often feel that it’s barely worth our time. That is where the danger of overconfidence becomes a menacing threat.

Usage Examples:

• "As we shall see in the next chapter, overconfidence can have pronounced negative effects"

• "Examples abound of the negative effects overconfidence has on forecasting accuracy.

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