I agree that a period of silence/prayer would be more significant for the family than flowers.

But whatever you end up doing, just keep it simple and sincere. The most important part of all this is the message you convey.
And regardless of whether the event gets press coverage...at least you'll have done a good thing. Something meaningful and loving in the face of the senselessness that caused this kid's death.

My condolences to his family and best wishes to everyone organizing the memorial. Hope it goes well. :)