Not to detract from your larger point, which is valid, but James and Lily were only 20 when they died. Harry started at Hogwarts 13 years after they graduated. It would frankly be weirder if there weren't a bunch of teachers at the one wizard school in the UK that had them as students. Hell, in the fortyish years that an actual, non-fictional teacher is likely to be working, they can expect to teach their first students' kids and possibly grandkids.