I'm much calmer than I was yesterday at this time. I spoke with our behaviour consultant and she is fairly confident that we can manage Alex's attempts to headbang on a window without increasing headbanging in general (or targeting the window specificially).
We need some information from the school first, in order to formulate a plan. Information like, how often has he targeted a window, is there a trigger/environment which prompts a window instead of another possible surface, is it a single headbang or is it part of a tantrum?
Once we have that, then we can figure out a plan for redirecting Alex to prevent him from targeting a window, without making him feel as if he's being thwarted in his intentions.