The OA Season 2: 12 Huge Questions That Must Be Answered
8. Who Was The Shooter?
Again, this may not be important. The identity of the shooter could be insignificant, because what matters here is Prairie getting to the school, the group working together - regardless of whether the movements really did anything - to stop him (it does at least appear to be a male), and then the impact of Prairie getting shot and perhaps dying.
Alternatively, his identity could be kept hidden because we know who the person is. Admittedly it's a slightly short list based on the number of white men we meet who aren't in the school already, but there's a chance this is a face we'll recognise, and that's why it wasn't shown.