낙제가 · 낙제를
failing (an exam or course), flunking
Example
시험을 너무 못 봐서 낙제할까 봐 걱정돼요.
I did so badly on the test that I'm worried I'll fail.
Word parts
낙 fall+제 rank
Literally 'falling from the ranks' of those who passed.
Good to know
Pronounced [낙쩨]; 낙제점 means a failing grade, also used figuratively.