A You raise an important point: how can teachers have high expectations of all their students if they never receive feedback about what happens to them in later life? Furthermore, an inclusive school community should be keen to keep contact with its old students. The private sector is noted for this kind of activity but with, one suspects, a rather different agenda.
State schools have been much less systematic in their approach, but for special celebratory events, many demonstrate that they can achieve similar contact with spectacular results in terms of positive feedback from and about old students, and have a lot of fun into the bargain.
If your school has had such an event recently, or anticipates one (or the pretext for one can be found), then you might suggest developing the system set up for that to create a regular point of contact, perhaps through an annual newsletter or event. The relatively modest financial outlay would be well spent in terms of good community relations and raised expectations.