Is that true see you later in classroom or see you later at classroom?

You would say "See you later in class" or "at class" (event)
but you could also say "See you later in the classroom." (place)

You need an article (the) to go with the object (classroom), but if you were talking about an event, such as "math" or "third period" you don't need an article (the). "Class" in the above case, is referring to something more specific: an event.

classroom refers to a place
class refers to an event

"See you later in the gym." (place)
"See you later in Gym" (event)
"See you later at basketball practice" (event)
"See you later, Alligator!" (silly rhyme)