синф: class, grade
My son studies in the eighth class.
Дар синфи шумо чанд нафар талаба мехонанд?
How many students are in your grade?
In Tajik you usually say what grade/class someone studies in, not what grade/class they are in. (In the US they say “grade”, and in the UK they say “year” or “class”.)
The word синфхона is classroom, but синф is also used to mean classroom.
The word синф is also used as the taxonomy class, for example the class of mammals is синфи ширхӯрон. You can see all the taxonomy classifications by looking at animals on Tajik Wikipedia, for example саг (dog).