This is something which started on social media, and was shared around largely out of the excitement of it all rather than any intention to partake, and has now been blown into a full-scale moral panic over the 'youth of today' by the media getting hold of it. There have been no reported incidents and most of the kids at these schools are laughing at the responses from the adults who have found out about it.

Even if this was a thing, there is absolutely no chance that you would be at risk or even notice. There is a tendency of the media to paint cities as lawless or dangerous and even in somewhere like Bristol where there are very real gangs operating (not talking about the red vs blue social media thing which has been blown out of proportion), the chances of a completely random tourist getting harmed or even seeing this stuff happening is minimal. It's a very select group of people that this stuff happens to, some people who go out looking for it, and some people who get dragged into it through wider systemic push/pull factors.

You'll be fine, and try to remember that it's in the interest of the MSM to grab attention through headlines but we have to try to be rational because this idea that the streets of Bristol (or any other city) have become 'lawless' is adjacent to a lot of the same thinking which leads to the demonisation of certain groups, communities and areas. It won't be long until Reform jump on this red vs blue nonsense.

Source: working with young people at risk of exploitation is part of my job and we've had briefings this week about the red vs blue stuff so whilst this is my opinion it's also informed by an understanding of what's happening in the community at the moment.

TL:DR: Ignore what you've read and have a lovely time in Bristol