The obvious solution is to work fulltime!
You don't want to because of work-life balance, then perhaps this is the price you have to pay. Unless you work in a small subject in a small school, then your decision to work part-time is your own choice.
It is fine going to your union and shouting your rights to your head, but is working to rule particularly helpful in the long run? That's a chance you take, I suppose. And you will be secure in the knowledge that yo were 'right'.
I never see my salary as an hourly rate. I have an annual salary and I do what it takes to do my job, regardless of the specific hours. I worked in industry for many years before teaching, and felt the same way. I never 'charged' the company for travel time or nights away from home.
In my 19 weeks holiday per year, I might spend the equivalent of 2-3 days on school work, which I really don't grudge. I'd rather not be chasing my tail at the start of each term, so it is better for me to work like this.
Clearly, it is difficult if you have small children and rely on babysitters, but these years soon pass. It is hard when you are in the thick of it at the time. In any work place that I have been in, these workers are cut some slack, but only because their colleagues step in and cover for them. You need to have willing colleagues who don't count their working hours to make the whole thing work out.