Could this also be "I have graded (the pupils' homework) now", i.e. I have marked it and assigned grades/marks/points?
You can use graddio in that sense, but then you would probably say what it was that you had graded:
- Dw i wedi graddio atebion y myfyrwyr - I've graded the students' answers
- Dw i wedi graddio'r cynnyrch fferm - I've graded the farm produce
Following from ibisc's answer, Welsh teachers don't really use "grade" like Americans - we "mark" papers usually, but will give a "grade" to GCSE work.
I'm afraid my school English is a bit of a mix of everything due to attending an International School where the English-speaking teachers came from a variety of countries.