If you're thinking that the schools could just offer their own email systems, have you figured out how much that will cost?
It sounds like you work as the school's email administrator. Since it sounds like you have a financial interest in the outcome of this, you should just be honest and up front about that.
Google has a remarkably good track record regarding security. They may be the best company (of Apple, Microsoft, Facebook and Yahoo) in their industry, and if they aren't #1, then they aren't far behind.
One of the issues you raise is that you are assuming that students will use Gmail for their personal and private use.
In fact, they are free to use whatever they want for their personal email, and simply configure their Gmail account to forward and delete after forwarding. I've investigated quite a few other email providers, and this is rarely a feature they're willing to offer, so in this respect Gmail is way ahead of the competition.
BTW, do you think the schools should also have to disclose that they're using Microsoft software, that it has a such a long and poor security history?