I'm an employer, OP (not education), and I have dealt with many flexible working requests. We grant most of them, but we often ask for compromises. The employee has rarely understood the true effect on the employer which, to be fair, is difficult to do because an employee often doesn't have a full picture of staffing issues.
I would avoid being too didactic about what your employer could or should do. Observations like "Why didn't you get maternity cover for me, then?" may not help your cause, given how hard and expensive it is to get suitable mat cover. You want to prime them to want to help you, and you don't want to make points that they can easily shoot down.
Use your union to approach this as a formal negotiation.