Experience with workload creep

I work at a preschool. It's not an English school but just has some English integration (aka me).

My first two years, there was one other English teacher that would come once a week to do scheduled English time for all classes whereas I would be there all day involved in all daily activities. I would do some activities in English usually once a week or so.

3rd year comes, regime change, they stop using the other English teacher that came once a week and I took over his slot on top of my usual. That was fine. I'm there anyway.

4th year I start having more English lesson times. 12 to be exact split between multiple levels. This eats up a lot of time and they never wanted to give me time to actually prepare so whatever. I do whatever I can and nothing more. I'm not gonna let a job stress me out.

5th year, same schedule. I ask for a raise, they deny the raise stating that I'm "the highest paid part time teacher." My response was, "okay. Pay everyone else more." It tickles me still when I meet a teacher from another school for the first time and somehow they know me as the guy that asked for everyone to get raises haha.

Starting this 6th year my lessons have increased from 12 a week to 20 a week.

For clarity, no lesson plans or things like that are part of my contract. Just general work stuff. I have also had the same pay for 5 years.

This is basically my English school at this point with none of the economic benefit. If anyone has experience with this, what are the odds I get something actionable if I have a chat with the workers rights bureau?

by itznotme