Being aware of our breathing is a big part of mindfulness. A settling activity could be for students to be quiet for two minutes and to notice their breathing. They can put their hands on their stomach or chest and feel it going up and down. Here are some ideas for other breathing exercises.
Hold your hand in front of you. Use a finger from your other hand to trace up and down your five fingers. As you trace up the finger breathe in, as you trace down a finger breathe out. Repeat.
Take two short breathes in through your nose so you look like a bunny twitching its nose, then take one long breath out through your mouth. Repeat. See if you can make up a breathing excercise based on an animal you like
Find a square shape in the room, a roof tile, window pane etc, and follow the four sides of the square with your eyes. As you follow the top side breathe in, as you go down breathe out, as you go along the bottom breathe in and breathe out to get back to the top. Repeat.
When you breathe in, think of all the good things you have in your life (friendship, love etc), and when you breathe out think of breathing out all the negative feelings (hate, anger etc). Imagine how the different feelings would look as you breathe them in and out.
Imagine the curly shape of a pigs tail, make it using your finger. Breathe in and out with the shape of the loop. Repeat.
Choose a number and take that many deep breaths. Or choose a number and see if you can breathe in for that long and out for that long.