Is it legal and if not what could be done after a school nurse take a urine sample from my 7 year old child and did not tell us?

I don't know of any reason why a school nurse would be required or for any school reason would need to take a urine test. I would be interested in hearing why they thought it was appropriate/permissible. So the answer here would require knowing that first. I've never heard of a school nurse taking a urine test. How was it administered? Was the nurse in the room when the sample was being given? Regardless, unless there is some civil rights or self-incrimination issue (which would be difficult to support probably), the likely negligent conduct by the nurse probably does not give rise to a worthwhile claim as one would need to show damages to the child on account of this conduct. Did the child need any medical, including psychological, treatment on account of this experience. If not it may be a case where the negligence is clear but there is no damage, so no case to pursue. In any case, you should get to the bottom of this with the school administration. This should not be allowed to happen again and all treatment and diagnosis done by the school/nurse must be relayed to the parents promptly. If I were in your shoes I would have a direct conversation with the Principal and the county administrators about this conduct and expect an apology and an explanation at a minimum. I am very curious to see why they needed this test performed. Good luck.