Cat, I agree that it is all about give and take. You do something above and beyond, and it gets remembered. So, when you need a favour, it is granted. Hopefully, you don't do it in a calculating, keeping score, way, but just when you can.
In my job before teaching, the company was really into a fashionable training course at the time called Seven Habits of a Highly Effective People. One of the key concepts on this course was of an 'emotional bank account', where you have to make deposits before you can make a withdrawal. Helping colleagues out (doing the green room during a Saturday evening drama/musical performance) would be a deposit, but this enables you to have some time off when they cover for you (attending an event at your own child's school). Working to rule makes no use of the emotional bank account and does not give you any benefit, as well as no downside.
I have always been of the opinion that if I am able to help a colleague or friend, then I will. The cost to me is usually repaid many times over. I like to keep a healthy balance in my emotional bank account.
If a school's budget is so tight that they can't afford to pay part-time staff for parents' evenings, then if the part-timer doesn't want to work, their colleagues have to stand in for them. If there comes a time when the part-timer wants a favour (eg to attend an event at their own child's school), why should their own boss agree?
We have a policy in our school that any kind of personal leave has to be covered in-department. If close colleagues are not willing to do that cover or jiggle their own timetables, then the personal leave is simply not granted. You have to keep your colleagues sweet. You don't know what is around the corner.
What comes around goes around.