They are the heart of the school. Without them who would input the attendance into the main computer? Who would answer the phone when half the school population is sick? Who would un jam the photocopier? Seriously, they are a pretty important bunch of people.
Be their friends.
If you have more than one prep (I say more than one because I believe even we subs are entitled to one prep a day if we get one - but more on this later) go and see if they need help.
I've photocopied for them, I've sorted for them, I've shredded the important documents for them (I did, I probably shouldn't have been near the important documents but that's just how much they trusted me).
Sometimes secretaries are the ones who call out for substitutes. Do you want to be invited back to the school? After you have won over the students win over the secretaries. It is a small investment of time that will go a long way. Besides, you are getting paid for the entire day of work - not to sit back with your feet up chewing bon bons.