I agree with the others that it's a bit too early to be wading in, assuming your child only went back to school last week.
Is he/she reasonably confident? We had a similar situation last year when DD was in Y1, and I encouraged my daughter to finish her work quite quickly and then go and ask the teacher what she can do next. This made her a ‘problem’ to the teacher, albeit a polite and positive one.
In our experience, the teacher did struggle to set appropriate work at first (actually, right until about February). DD would come home saying she’d helped several other people once she’d finished her own work – it felt like she was being used as a spare TA! Of course, this was beneficial to embed her learning, but there was a bit too much of it, and we were worried other children might start to resent it. DD also did a lot of reading in class to fill time. Eventually the teacher set additional ‘challenge’ activities in maths which helped somewhat, though I wasn’t convinced she was really stretched all that often.
Another option is to encourage your child to stretch his/herself. If given a maths problem, for example, encourage him/her to solve it first, and then invent his/her own extension (bigger numbers, more variables, more stages etc). I know that’s a big ask for a Y1 child, but may be worth a try.
Hopefully, once the teacher has got the measure of the class, and has seen that your child is capable of more challenging work, things will improve. I’d say give it a couple of weeks, and if your child is still saying the same things, ask for a chat with the teacher.