I reckon that it's up to you. She left school when her Mum was killed, and did not go back that year. She was at the battle. My solution (for "Fred and George's Busy Day") was:

‘Shouldn’t you be back at school?’ Fred asked Hannah, grumpily.

‘I’m not clever enough; I only got five OWLs and I didn’t take my re-sits because I left when the Death Eaters killed my Mum last year,’ Hannah declared bitterly. ‘I’d be a seventh year retaking fifth year lessons. I’ve been working here for ten months, and Tom knows people in the Ministry. He persuaded them to make an exception for me. I can do more good here than I could at school.


ps by extracting that quote I've discovered a missing speech mark.