In Mexico it's very similar to Spain, and works like this:
Year
1
2
3
4
5
6
7
8
9
10
11
12 |
Grade (Mexico)
Primero de primaria
Segundo de primaria
Tercero de primaria
Cuarto de primaria
Quinto de primaria
Sexto de primaria
Primero de secundaria
Segundo de secundaria
Tercero de secundaria
Primero y Segundo de prepa(ratoria) (La prepa va por semestres)
Tercero y Cuarto de prepa
Quinto y Sexto de prepa.
|
Grade (US)
First Grade
Second Grade
Third Grade
Fourth Grade
Fifth Grade
Sixth Grade
Seventh Grade
Eighth Grade
Ninth Grade
Tenth Grade
Eleventh Grade
Twelfth Grade |
So something similar to "I'm in 9th grade" would be "Voy en tercero de secundaria".
If you want to be more specific, remember that
primero, segundo, tercero, cuarto etc., aren't referring to grades (grados)-- they refer to the year (for primaria y secundaria): "Voy en tercero de secundaria" = "Voy en tercer año de secundaria". For the prepa, the ordinal numbers refer to the semester you're in. So "cuarto de prepa" would be the second half of 11th grade.