I've got an 11yo DD as well. She occasioanlly throws a wobbly (ending in tears) in situations like this, and yes, mainly at night when she is over tired. I can be a perfectionist too at times and I think my DD shares this trait so I have to make sure I react calmly to show her that difficult challenges are best dealt with calmly. I find that she tends to mirror my mood and if I start to raise my tone at her frustration the whole thing just escalates so what I try to do (not always successfully) is to distract. If she's in bed try just walking in, sitting on her bed, not raising the issue about her handwriting etc and instead telling her a story about a frustrating incident when you were her age. Don't let the conversation turn back to tonight's situation just end with something totally unrelated like weekend plans... if she wants to talk about the assignment say that you'll look forward to talking about it over breakfast becasue nothing can be done about it now. Be firm with this. Learning to put aside daily issues so that you can sleep is a valuable lesson in itself. Sorry if I sound preachy, I'm just sharing what works for me. However regarding the actual problem; speak to her teacher. They should be encouraging the students to enjoy the learning process and not put too much pressure on about things like handwriting. Sure writing is important too but how come she isn't more pleased with herself about getting all her spelling words right? Some teachers can get priorities wrong and the more demanding skill of accurate spelling should be focused on (an applauded) more than handwriting, especially if the writing is legiable. This is something my daughter's school has actually taught me (the perfectionist remember!) number one priority is to establish a love of learning, everything else will follow. My DD attends a private school though... and I too suffered more from perfectionist teachers when I was a child in the public system... not meaning to make blanket statements... just want to encourage you to speak to the teacher to make sure all's well. How long did she have to do the assignment? Could time management be the biggest issue?

Good luck!