You'd normally use the verb 'haber', equivalent to 'there is' or 'there are', in this instance. This site explains the sometimes subtle differences: http://spanish.about.com/od/usingparticularverbs/qt/haber_vs_estar.htm In English I might say" In my son's class (there) are 25 students.' I don't mean that right this minute, at 6:45 in the morning, there are 25 students there at the school in his classroom. I mean that there are 25 students assigned to his classroom. If this is the meaning you want, you need the verb 'haber'. I suspect you want to say 'En mi clase hay 10 estudiantes'.

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"

I prefer to say "Hay 10 estudiantes en mi clase". That's better.