Assuming you are in Bucks given you mentioned the 121.
As a general rule, the “top” 25% of Bucks children will pass the test. Arguably this includes a number of children who have been so heavily tutored they manage to artificially elevate themselves (but then struggle once they’re there), but on the whole the brightest children are the ones who pass.
You must have a vague idea of where your daughter sits in her cohort. It sounds like she’s way off the top of the class, but where does your gut tell you she sits? Higher end of average then you have nothing to lose in trying. Slap bang in the middle or below and it might be a step too far.
Without an EHCP or medical diagnosis then there will be no adjustments made for her in the testing process. Her current school will have to submit “suitability for grammar” scores prior to the tests. These are used in cases of selection review. Look at the relevant section of the council website for the detail.
www.buckinghamshire.gov.uk/schools-and-learning/schools-index/school-admissions/school-admissions-guides-policies-and-statistics/guide-to-grammar-schools-and-the-secondary-school-transfer-test-11-plus/selection-review/
I would recommend a meeting with her current school to go through how they would most likely categorise her based on her current academic performance.
Please be realistic. SEN or no SEN would your DD actually thrive in a grammar environment? Not just be ok, but thrive. It’s so hard when you already have one at grammar to not want the same for your other child. This can put an awful lot more pressure on the second child though and make the fall if they don’t pass much larger.
Sorry, I waffle. I don’t think it’s as clear as just using “expected” as a guide. Is she really working at the top end of her cohort, can she cope with the fast pace of learning at a grammar, and how would she feel if she was always towards the bottom of the class in an academic school?