You could actually say "10 estudiantes están en mi clase". That would be correct and it would mean something like "Right now there are 10 students in my class (but there normally aren't)". The correct way to say would be "Hay 10 estudiantes en mi clase". The verb haber here is used to "show" something is somewhere as in "There is a dog under the bed" or "There is apple on the table". "Hay un perro bajo la cama" / "Hay una manzana sobre la mesa".
If you say "El perro está bajo la cama" you are saying "The dog is under the bed"