( i think I deleted my first post before it posted! hopefully you don't get this twice!)
I am not a school nurse. I precepted with one for my peds rotation (I was shocked how many kids flow in & out in one hour!!!) I do have 2 boys that have been to the nurse's office plenty!!! i hope these suggestions will be handy.
Refer to the school's policies. They must state what definate s/s will send a child home. If policies are not available, you could probabaly find professional organizations dedicated to school nursing on line that outline standards of care. As the school nurse you are responsible for educationg the staff as well. Email all teachers (so as not to single out!) the most updated policy or standards. Also, network with other nurses in that school system or state.
Assess the child, document findings, act accordingly. If the assessment does not render a ticket home, send the child back to class. Explain to the teacher the the child's (lack of) s/s do not meet criteria set by the policy or standards of care to send him home. Maybe a phone call home to the parents for a head's up that the child was in your office might get you some clues. For instance maybe mom will say he's been eating less lately or just as active as ever. Get the parents involved. That way it becomes parents -vs- teacher, not coworker -vs- coworker. Your job is to assess... not diagnose (I had this made quite clear to me in my rotation).
Unfortunatly you two just may butt heads on this forever. Taking your concerns to the principal may also be helpful if this becomes persistent. Stick to your guns, after all you are there for the kids. Keeping them in school is the main goal. Be armed with policies and documentation that you practiced within those policies and you should be fine. Also, brush up on communication skills...it sounds like this may take alot of time to resolve.
Keep up the hard work, schools
can't function without you!!!