I have a ds at Watford Boys, had a dc at another local state, and have friends with children at all of the local state and private schools.
I am very happy with the Boys School. I haven't heard any stories about bullying or gangs (not saying there aren't any, but I haven't heard any mention at all from ds).
Academically, it's solid. Good ethos, teaching is generally good. It has a fantastic music department, which is why my ds is there.
And, yes, I have many friends who sent their dcs to MTs or Berko or RMS because they didn't get into the Girls or Boys, so that is definitely a thing around here.
I don't like SCDs. I know many dcs who did experience bullying, and my two were adamant that they didn't want to go. Again, just my experience.
Saying all of the above, my eldest dc went to another local state secondary by choice, and is now at oxbridge.
Guess the answer is, don't overthink it tbh.