1. Let all the pilots know that is better to avoid this school
Why is that, exactly ? Are they not honouring their contracts with clients ?
By the same reasoning, you can argue that we shouldn't fly Ryanair, that we shouldn't drink tea or coffee unless it has a fairtrade seal, etc... But I don't think that makes a convincing argument.
2. I know that if the ownwer read this maybe think uhhh is better to follow the law because apart form 350 euros a month in the contract appears that they only work for 5 hours a week. So he pay less
What you are claiming is that the owner pays 350 euros a month, gives people a contract that states they only work 5 hours per week, and then forces them to work more ? Any concrete evidence of that ? Are you talking from experience ?
3. Soon I will post a letter in english and Spanish that we can post to the work inspector in anonymous way. So I think if he receive 50 letters he will have a look to the school and maybe this situation will solve.
I'm sure a Spanish civil servant will be thrilled to receive 50 anonymous letters. I hope you're not wishing he or she will act on them.
What you could do (and which I advise you to do) is write a properly signed official complaint letter, stating your experiences and attaching your evidence.