meaning - Present perfect with since interpretations

I have a couple of present perfect + since examples below.

1. I have studied two books since 9.15.


2. We have taught at this school since 1965.


3. They have been at the hotel since last Tuesday.

Sometimes the meaning is clear and sometimes it's not.

1. Am I still studying more? How is it clear?


2. Are they still teaching?


3. Is clear if they are still at hotel?

I would like some explanation for these questions on what do they mean and why do they mean that?