Datun, surely the first step is to have clear policies, checks and procedures so that the likelihood that anything untoward happens is kept to a minimum.
I assume that since this person is a teacher they have an enhanced DBS. Just because someone's spidey senses suggest someone is dodgy, it does not mean they are. And the reverse. Lots of evil has been carried out under the cloak of respectability. But the potential for problems must be kept to a minimum.
Prism should have a clear appropriate safeguarding policy. Given they cater for quite a wide age range, thought should be given to older attendees. For example should 16+ attendees have DBS checks? The policy should be displayed on the website, as should the name and contact of the person responsible for safeguarding, and ideally the governance of the organisation and the names of the people running it.
I am not an expert in safeguarding, and someone like R0wantrees would know exactly what is required. However what Prism have is clearly not good enough. If someone was aware of a problem, perhaps including this poor use of social media, how would they know how to report it.
And what the hell are the police and others doing supporting Prism when a quick glance at their website gives no evidence that they have considered safeguarding or done anything to minimise risk.