I’m a teacher. I wrote a rec for a student who switched high schools. He had to set it up with his new school because they had a slightly different process for using Common App, but after a bit of back and forth we figured it out. I would had written him one had he stayed, so writing one for him during his senior year was no problem. I did check with my administrator first, to be sure there was nothing problematic. Easy peasy and totally fine; it happens more often than you’d think.