will attending summer school help ur application?Watch
YES. A couple of examples of topics you did at the Summer school and a mention of how it helped your confidence, and your determination to go to University - exactly what a Uni is looking for. And it will really help your subject knowledge for Year 13. All in all - a good thing to do.
If you are applying to UK universities, I wouldn't recommend paying for one. It will help your application, but so will many other things.
However if you're from a widening participation background, and it's a free summer school offered to you then YES — do it!