Speaking as someone who has done internships for 7 different companies (we call them co-ops here in Canada), perhaps this feedback would have been more valuable

Speaking as someone who has done internships for 7 different companies (we call them co-ops here in Canada), perhaps this feedback would have been more valuable for both parties if it was disclosed to the intern privately during his work term as he was making these mistakes. Many of the social/workplace norms you might take for granted in a new hire are definitely not obvious for someone entering the workforce for the first time in his life. Speaking from personal experience, a bit of patience, empathy and well-meaning advice would benefit these junior developers much more than ranting on a public medium with a tone that's obviously full of bitterness and spite. You will also end up getting a much better employee in the process, both during the work term as he improves his behavior as well as in the future when he may return to work for you full time.