I put my hand down the seat of the school bus and got pricked with a needle. What are my chances of getting hiv?

Needle stick risk. Because you do not know the etiology of this needle you would have to consider it the worst case scenario. The risk of HIV infection from a needle stick is 0.3%. Other things are much more catchy however: hep b 30% hepatitis c 3%. You should report to your supervisor who should send you for post exposure prophylaxis.