You did exactly what you should do in providing care for your daughter with TypeOne Diabetes. It appears that the school might be acting contrary to United States law.
Public Law 101-336 [known as Americans With Disabilities Act of 1989] specifically allows for care, treatment, self treatment of persons with TypeOne Diabetes and for reasonable accommodation of and for persons with T1D; T1D is the only “disease” specifically named in the Act.
Under no conditions should you administer her insulin [or do a finger-stick BG check] in a public restroom, an unsanitary place; that is a location often times used by unsavory characters ‘shooting up’ purposes. Bringing her to a nursing station could be acceptable, but it also could deprive her of continuous and full participation in her class activities - something the law intended to eliminate. Bottom line, your daughter should NOT be taught to hide her condition and she must not be treated as a pariah.
Perhaps you could contact the JDRF and the American Diabetes Association for help in education this public school. Your local JDRF chapter may also have an outreach person who could visit the school.