That sounds tough. Do you share a class or are you same year team but different classes? If you share a class, it's obviously more difficult.
The good news is that the headteacher can clearly see through her!
If she doesn't want to work evening/weekend, that is, of course, absolutely fine but she needs to ensure the tasks are completed during working hours.
Are you her line manager/phase leader or is there someone above you?
Either way, I would make sure you keep records of absolutely everything.
So, if you verbally agree that she plans English and you plan Maths, follow up with an email 'further to our discussion, I am just confirming that you are doing x and I am doing y by a date'.
If it isn't done, again email, 'we agreed x would be done by y date, please could you send me x as agreed'. Keep it factual and ensure you agree deadlines every time. You may wish to copy the manager in, but if not, at least you have a paper trail.
Give it a couple of weeks when you are back if there is no improvement, arrange a meeting with the head or line manager to discuss things.
Never easy but you can't carry someone.