The purpose of the EU Directive is to make sure people get at least their minimum amount of holiday and the employer cannot buy back the holiday.
Given that you get this during the school holidays, I cannot see there is an issue.
Your contract, which you have agreed to, merely constrains when your holiday is taken. The uplift in pay has compensated you for this.
That is the logic of my answer; I am only 90% sure on this one, so others are welcome to correct me.