I sometimes fall asleep while studying, even with ample sleep. However, I don't fall asleep while engaged in any other activity. Is this narcolepsy?

No. Most likely lack of interest , when u feel like that take break from studying for few miniutes and do some excersize to increase u energy.
Unlikely. Narcolepsy is a very rare condition. The most likely reason to fall asleep is lack of sleep or possibly in your case boredom. Narcolepsy can present with cataplexy (sudden loss of muscle tone WITHOUT loss of consciousness brought on by emotion), sleep paralysis, sleep onset hallucinations and poor or disruptive sleep. if these symptoms start or your sleepiness gets worse talk to a doctor.