Just because one becomes an adult does not mean that an adult is less deserving of the same level of protection.
If using Google is mandatory for a class, then the students don't have any choice in the matter. And I think school computers are locked down to prevent installing browser extensions, so that option isn't available either.
Also, because it's a pet peeve of mine, it raises the question.
There's really no difference. It's all about network effects. We're seeing Google docs, Dropbox folders replacing what Microsoft Office was in the past. Vendor lock-in by network effects.
You may like it or not yourself, but let's not pretend it isn't something that it is.
It's why some countries have banned certain advertisement mediums targeting children, and why you won't see many tobacco billboards by schools.
I suppose one could choose not to use the Internet in that case but that's not really a viable solution.
It's an asymmetric fight which individuals are on the losing end and given what we now know post-Snowden, I'd argue that greater privacy protection is absolutely necessary and available to all those who desire it.