I know it's the law, but really, is there a school in the land that sticks to the rules? I think not.
I know it's not right and I know that in an ideal world, there would be a qualified subject specific teacher on call, for every school that needs cover, whenever they need it, but that just ain't gonna happen. All I'm saying is, that at my school, a teacher can call in sick and not have to worry about what is happening whilst they are not there. We do it for the love and support of our colleagues! 
My CS's are under no illusions that they are just glorified babysitters and TA's with a posh name, but it's about supporting our teachers and if that means 'teaching' a lesson, then they will do it!