fact: as a diabetic your are identified in the americans with disabilities act as having a disability that is protected by federal law from discrimination. therefore, the school is required to accommodate you in the classroom and to not force you to "deal" with it outside the class. so, if everyone is aloud to breathe in the classroom, the must allow you to test bg, as it is a required and normal life activity to live w/o part of your pancreas!
start testing in the classroom when it is necessary, and if asked, tell them that you cannot leave the classroom until you are sure you will not endanger yourself by walking to the nurses office.
if they hassle you, contact the federal gov and file a claim of discrimination. name the school, the principal, the superintendent , president of the school board and the teacher.