Mainly a lurker, but thought I'd post on this as I've recently had this discussion with some Heads.
The Google statement says (slightly boldified for quick reading)
Additional Services require consent for minor users: G Suite for Education requires in its agreement (section 2.5) that schools obtain parent or guardian consent for any Additional Services they allow students under the age of 18 to use.
Schools may opt to get consent for Core Services as well: As a best practice, school may also want to get parent or guardian consent for the Core Services they enable. Otherwise, a school would provide consent for Core Services on parents’ behalf by signing up for and using these G Suite for Education services.
Being as Classroom is a core service, it will be fine - however, our MAT has opted to get parental consent anyway as it is indeed best practice. If any parents do not want their child(ren) using Google services, we have alternative provision in place.