Get an idea how much force it takes to squeeze blood out of the heart....
The force needed to squeeze a tennis ball is similar to the force needed to squeeze blood out of the heart.
Gather enough tennis balls so you have one ball for every two students (if you can get one for every student, they activity will proceed a bit faster, as you won't have to repeat every step). Ideally, you'll find someone who plays tennis and can get you as many dead balls as you could want - your phys. ed. teacher might be able to help you find someone.
Repeat for the other students.