Their success is not your problem, you have to learn for you, as well as you can.

I use to tutor friends. I still remember in high school, my best friend would sneak me into her house, she wasn't supposed to have visitors in the evening on weekdays, and I'd tutor her; we had the same teacher so were on the same lessons. But I never did my homework or studied so she actually did better than I did on tests because she worked at it whereas the only time I looked at the material was when I wasn't daydreaming in class and tutoring her, I'd figure it out (but not remember it; trig :-)

Everyone is more complicated than just learn/don't learn, pass/fail. There's lots else going on in a person's life that we can't begin to understand (look how well we don't understand ourselves :-) My friend was over-achieving and I was under-achieving but the "reasons" are way too complicated to be able to understand someone else.

Do the best you can; it won't always be that easy and getting in the habit of just vaguely studying to get by can turn around and bite you in the future (been there, done that). I got 15-20 years down the road before I realized I'd screwed up and not taken the "right"/enough math at University to be able to do/understand what I wanted to try.