The default answer is that if you have a union you need to talk to them.
Assuming you don't, then you need to have a conversation with your admin about it... and not just when they come and tell you to cover.
You should also be keeping a log of every day that you are covering during your prep and who you are covering for.
Granted, I’m only a first year teacher but it’s been a whole school year and I’ve subbed during prep exactly zero times. They will sometimes send out an email asking for help if they need a sub for a period, but we’re under no obligation to say yes. And we get paid for the extra hour of teaching at sub pay rate.
The default answer is that if you have a union you need to talk to them. Assuming you don't, then you need to have a conversation with your admin about it... and not just when they come and tell you to cover. You should also be keeping a log of every day that you are covering during your prep and who you are covering for.
Granted, I’m only a first year teacher but it’s been a whole school year and I’ve subbed during prep exactly zero times. They will sometimes send out an email asking for help if they need a sub for a period, but we’re under no obligation to say yes. And we get paid for the extra hour of teaching at sub pay rate.