I don't think removing the encryption on communications in order to monitor children's activity is teaching anyone to be or being a responsible digital citizen. I know if this was asked of me I would switch my child exclusively to 4G and make my child aware of the issues that surround a mandatory backdoor of private communications demanded by those in authority.
If the issue is objectionable material on students' devices then that can and should be dealt with on a case by case basis by parents when advised by the schools.
I would like to clarify that when performing SSL decryption, the traffic does not have encryption removed. The traffic between the user and the website is still encrypted and private.
So in the example where traffic is delivered over HTTP, it is possible to capture the stream at any point between the user device and end website and this could allow the snooper to view the information such as details posted in a web form. (no privacy of information)
With the SSL decryption feature used, the traffic flow is the following;
User <encrypted stream> Proxy (scanned) <encrypted stream> Website. So at no point can someone capture the traffic and view the stream as you can with HTTP, therefore the privacy of information is still protected from prying eyes. (which is what SSL encryption was designed for)