I am native AmE, so this is speculative. As a practical matter, in everyday conversation, "What grade is X in" and "Which grade is X in?" are probably used interchangeably more often than not. There is a slight difference, with the "what" version being a more general question and the "which" version suggesting selection of one grade from a specific group of them.
I think that difference more or less matches up with the "what/what kind of" feeling of jaký compared with the "which (out of a group)" feeling of který. But, speaking only about the English side, while either "what" or "which" might be more appropriate in a specific situation, very often there is little practical difference between in daily use.