How dangerous is it to not report a concussion? I don’t want to miss out on my sports season.

Brain injury is one of the leading causes of death among athletes. High school athletes are one of the most at-risk groups, with football and soccer players at the highest risk.
A concussion occurs when a blow to the head forces the brain to rock violently inside the skull. Severe or undiagnosed concussions can increase the risk of permanent brain damage. It is important to stop playing immediately and have your head examined by a doctor.
After all, your primary concern should be your health, and not about missing practice or another game.
To learn more about concussions:
http://www.mayoclinic.org/diseases-conditions/concussion/home/ovc-20273153 https://www.cdc.gov/traumaticbraininjury/symptoms.html