If there's a problem, the school/ teacher should be overseeing more. It's the teacher's job to make sure the student is coping and covering everything properly.
But it doesn't always happen.
My first ever placement was 3 weeks in a year 3 class. I was only meant to take the class on my own for maybe half an hour a day - rest of time was alongside teacher. But after a couple of days the teacher buggered off ill and I was left with the class for the whole 3 weeks. I was woefully unable to cope. I felt horrible and inadequate at the time, but in later years, angry. They nearly jeopardised my chances of becoming a teacher by using me as an unpaid supply. It was grossly unfair on the children, too.
Fast forward to my final placement with a year 5 class. I did a whole term in that class and I was absolutely fab, probably the most enthusiastic and motivated I've ever been. I was so sad to leave and I know the children did really well that term, probably better than with the class teacher who was much more jaded with it all. Students can breathe new life into a school.
In later years I had students in my own class. It's much harder than it looks! You are basically mentoring them and should be overseeing everything they do, reporting back to their tutors, guiding, helping with planning and evaluations, whilst still making sure all your class are still on target. Certainly at primary level, it's harder work than just teaching the class yourself!
If you have concerns, speak to the school. Nobody should be using having the student there as an excuse to doss. If it's a problem with this particular student, far better to mention it than have this person let loose on their own class one day if they aren't yet ready, or suitable.