Infringement would be using a trademark, breach of copyright or passing off.
the phrase Britains "got talent" would not be protected by copyright as it is too short and too common a phrase . I very much doubt it is a trademark which leaves passing off.
This is where a phrase is used to suggest that the product is that of the famous manufacturer and associated with it.
This is clearly a school talent show and I doubt that anyone in their right mind is going to turn up expecting to meet Simon Cowell.
As such you should be fine.