Once I got moved into an 8th grade class an hour before school ended because the teacher got ill after lunch.
It felt like half the class had some kind of complaint or tattle as soon as I walked in and there was no assigned work.
I handed out some crossword puzzles and lined sheets of paper.
I told them to work on the puzzle and also write their regular teacher a letter telling her how they behaved after she left. Whenever a student threw something, poked someone, dropped items repetitively, etc I told him/her to make sure that was written down. Of course when they did something nice like share pencils, open the window, work quietly, help collect papers, etc I had them write that down too.
Amazingly it worked! They were so busy policing themselves that the hour just flew by and I didn't have to deal with tattling
I used to bring age appropriate distractions when I subbed - coloring sheets and fun stories for the younger kids, crossword puzzles and word searches for the older kids. These were really easy to find on-line.
Writing names on the board is surprisingly effective also - as long as you give them a chance to behave and get the name taken down as well.
Good luck!