Had to look up what s5 and s6 are. I think they are old enough to find "porn star pseudonyms entertaining at teacher's leaving, without the being corrupted. It may not have been the wisest of casual activities for a professional to give, but nothing to make that much fuss of. A quiet word should suffice.
I think he behaved in an entirely unprofessional manner, no matter what the ages of the pupils. He deserves his reprimand. A quiet word would not have been enough.