Finally...justice!
I invited Bossman and Bosswoman into classroom 14 for a private chat this afternoon. I listed my accomplishments in the past year, other good things I've done for the school and also the results of recent feedback forms, and asked for them to consider raising my number of holiday days from 25 to 30 as compensation. Without hesitation, BM looked at BW, they nodded and then BM told me that they didn't need to consider it, and they approved it on the spot.
We talked about other organisational things for about 20 minutes, and then as we got up to leave, BW smiled at me, and said to BM, 'Tell him.' I was puzzled. BM smiled and then told me that at the beginning of April they had decided to give me a raise (I hadn't asked for one), and for me to expect my salary to be higher this month! I thanked them both again.
I came out of the meeting and Young Secretary said to me, 'Can I be nosey and ask what that was about?' to which I sternly replied, 'No.'
Just before I left school, I found out (through the grapevine) that Incompetent Colleague had asked BM for, and been denied, a pay rise yesterday.
************************ UPDATE ************************
Something went wrong with the bank transfer for IC's salary, and I overheard him talking to BM about the correct amount that should have gone in. It turns out that, with my raise, I take home €125 a month more than IC.