It depends on what you mean by "grade"
If you are referring to quality, such as "what grade is this cut of meat?" or "I don't care what grade you buy as long as it does the job." The Spanish for "what grade" is "Qué grado?"
If you are referring to an academic grade describing how many points you have on an exam or similar, the Spanish for "what grade" would be "Cuál nota?"
You could say "el quinto grado".
Estoy en el tercero grado (I am in the 3rd grade)
el cuarto grado
You can say "Ella era en el sexto grado."
estoy enfadado/enfadada (girl)
estoy en el grado 5
third grade... tercero third year... tercer ano
Grado 3, seccion A, pulilas espanolas.
Te he conocido desde el cuarto grado.
a grade
Yo estoy en sexto grado. I am in sixth grade.
Soy feliz cuando puedo obtener una buena nota en español.