I work in schools.
I work in UK schools.
I work in IT in UK schools.
This is normal. Sorry, but there's nothing shocking here.
You join our domain, we get the right to push any and all security measures to your client that we deem necessary. If you don't want to allow it, don't join our domain (which also means we probably won't authorise you to use our Internet connection, etc.)
The domain will have a "Default Domain Policy" that almost certainly includes software you don't want (but we insist you have), settings you'd rather not have (but which we will enforce on you) and things like this - installation of a required domain certificate so we can check your not using OUR SCHOOL FILTER to do illegal / illicit things.
Chances are if you read your network acceptable usage policy, it states this. The alternative is you don't get network access. Because we are LEGALLY RESPONSIBLE for what is accessed through the network on our network, as well as the protection of our internal data and services.
Complain all you like. The alternative is that we block SSL site-wide. That means no Facebook at all, by the way. Or GMail. Or Hotmail. Or anything else that uses SSL by default.
We have a legal duty to monitor, record and analyse the logs of Internet traffic to ensure our child-protection policy (a legally-required policy) is followed. Additionally, it's OUR resource. If you want to use your own external 3G connection on your own time, argue for that. Chances are it will fail.
If you want to use the SCHOOL connection on SCHOOL time for NON-SCHOOL business, that's not going to happen. However if you want to use it for SCHOOL BUSINESS then you are required to allow us to apply our domain policy. If that, at any particular place, happens to include SSL certificates, monitoring software (potentially even INVISIBLE monitoring software like Securus, Ranger, etc.) then that's what you get.
Sorry, but as an IT Manager specialising in schools, and working in state, private and boarding schools from primary to further education, this is bog-standard and has happened for years. I believe even places like LGfL (a London-wide, government-backed school IT services supplier) do it.
There's a reason - we are required to protect our systems and protect ALL the children. That means everything gets summarised, logged and monitored. If we then need to dig into detailed logs, we can enable that option and do that too. Because - as in a previous school I worked for many years ago - we get things like members of staff browsing child pornography on school time. Yes, they are that stupid. And yes, they get caught. And, sorry, but our child-protection and data-protection policies take precedence over you going on your private Facebook after hours and we can't spend the time to distinguish hours, locations, staff-types, etc. for everyone.
If you don't like it, do not join your computer to a domain. If you are on the domain, it's literally our DOMAIN. Our rules. Clearly stated. That you would have agreed to.
Please, also don't act like your the first person ever that this has happened to. It's been standard practice for at least the last 15 years I've been working IT in schools in the UK.