I work for a school. My traffic is not MITM'd, but the kids' traffic is, because we don't want them using their school-issued laptops to play games or go shopping, and you can't adequately block stuff if it's all encrypted.
This is really hard to do in practice: for example, if you block YouTube.com you just broke a ton of lesson plans which rely on students watching things like scientific materials from NASA, HHMI, etc. It turns your approval process into a source of political blowback unless it’s really fast, and it’s usually not a good idea to be in your users’ minds negatively all the time.