Does a school have the right to prohibit students from attending classes because of certain clothing?

At Taras Shevchenko Gymnasium No. 109, the principal did not let children to class because they were wearing capris, which she considered to be shorts. The parents contacted the police, and the educational ombudsman Nadiya Leshchyk stated that the requirement for school uniforms was illegal. Can I, as a parent, appeal the actions of the principal of the gymnasium if I believe that they violate my rights and the rights of my child?