Sorry to say but this is old news. This has been happening in Spain at other schools already. People earning under minimum wages or even working as freelance when in fact they were full-time employees.
This will only end when either authorities or unions (i.e. COPAC/SEPLA) take measures, and I don't see it happening. SEPLA tried something a couple of years back at Airmed but the result was that instructors that made noise were fired anyway and those who wanted to keep their jobs had to accept very low T&Cs.
To make things worse, as someone cared to explain (I did also in a post in another section of the forum) with the current economic situation and unemployment in Spain earning 400€/month as flight instructor may be better than having no job, or earning similar figures working at McDonalds flipping burgers.
I'd rather be flying, at least you keep yourself in the loop and fill up the logbook (and in my particular case I loved teaching, a lot more rewarding than other jobs)