in the bedroom.. in the classroom at the party, at home.. why in and at are used differently?

~~Thanks for the question Mr. Mingky,

Here's the explanation to your question:

We use  In  for nonspecific :

times during a day, a month, a season, or a year.
inside a big place (towns, counties, states, countries, and continents).

EXAMPLES:
  She likes to jog in the morning.
  He's going to quit in August.
  It's too cold in winter to run outside.
  He started the job in 1971.

We use  AT  to designate specific:

times, adresses, and places.
in a small place.

EXAMPLES: 
   The train is due at 12:15 p.m.
   I live at 55 Boretz Road in Durham.
   Jane is waiting for you at the bus stop.

 Moreover, ( In = Inside )

"Sam is in the house."
->  We know from this sentence that Samand is inside the house.

"Sam is at the house."
-> We cannot tell from this sentence if Sam is inside the house or outside the house but we know he is somewhere at the house.