Slughorn goes to Hogsmade for collecting reinforcements.
According to an answer on Quora, JK Rowling said in an interview last January 2008 that
the group who followed Slughorn from Hogsmeade to Hogwarts to fight for the school were mainly Slytherins and their families, because Slytherins are brave in a practical way and so they went home first and collected reinforcements before joining the fight. She says that this was one of her favourite scenes in the book so it’s unlikely this is an ex post facto rationalisation, But she forgot to specify that they were Slytherins in the book.
In the book they were no Slytherins who fought. Since she is the author, is this true? We don't know if those reinforcements are Slytherin because it's not in the books nor in the movie.