Painless hard bump on hard palate along the midline to the left. hardly seen in a mirror, but can be felt with tongue and finger. what could it be?

Could be exostosis: How long has it been there? Is it growing rapidly or slowly? It could be a palatal torus, but without visualization, it is hard to say for sure. Have your dentist take a look. http://en.wikipedia.org/wiki/torus_palatinus.

First guess is a : Small maxillary midline torus which is a bump of bone. Harmless. But that would have been there for a few years at your age. Minor salivary glands can also swell and make a palatal bump. There are other causes too. Please let your dentist, oral surgeon, or periodontist take a look to be sure. If this is a soft tissue lesion it needs to be examined.

Exostosis: It could be normal bone we call a torus. It could be an inflamed gland or a localized abscess. Have a dentist check if for you.