Who attacks Jem and Scout on their way home from the Halloween Pageant at school? Who saves them?

Ewell attacks them. Boo Radley is the saviour.

Scout and Jem are attacked by Bob Ewell. This is Bob's revenge for Atticus defending Tom Robinson and showing Bob to be a liar in court. Fortunately for the children, Arthur 'Boo' Radley shadows them and intervenes when Bob attacks, killing him in the process.