There are two questions here.
The easier question is describing volume. There are two separate ways to measure volume. In metric terms, one standard measure would be liters and the other would be cubic meters. They are related in metric: one liter is 1000 cubic centimeters. Both of them (and their "American" counterparts) are extremely common and important to know about.
The other question is how to confront a teacher who isn't really on top of the material. This looks like an example of inquiry-based education, where the students have to think about examples in their life where they measure volume and then think about it to generate their own knowledge instead of passively getting facts from the teacher. I think it's a great technique and one that is rightfully gaining popularity in education. The downside is that some teachers think that you can just hand out a worksheet and kick back while they fill it out. In reality, teachers have to work even harder to engage individual learners to think for themselves if they aren't sure how to engage with a lesson, and probably not all teachers are trained in how to do that.
Certainly, your first approach should be to talk to the teacher. Talking to parents is a positive on our year-end review! You might get a sense from there of how high up the problem goes. Maybe the math department at the school doesn't care, or the entire school, or the entire district, or the entire state. And going to a principal with a solution is definitely better than going to a principal with a problem, if you and the teacher can figure out how to make things better but it requires extra training or resources or whatever.