estoy en el grado 5
You could say "el quinto grado".
You can say "Primer grado es dulce" in Spanish. This phrase captures the idea that first grade is sweet or enjoyable.
5 in Spanish is "cinco". You cannot say "five" because that is an English word, not Spanish.
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?"
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)
third grade... tercero third year... tercer ano
Grado 3, seccion A, pulilas espanolas.
cinco millones
Condom!! but you have to say it really fast 5 times :)