So found a piece of writing in my son's book bag after he went to sleep. It read(without names) :
Plan a: catch child x, child y, child z
Plan b: kill miss teacher
Plan c: escape from my house
Initially I read it and thought it was probably just a silly game or something but then I remembered he said he felt sad at school today but wouldn't elaborate. Will obviously have chat with him tomorrow. Just up worrying about it now!