Do you mean a true comprehensive (that take and cater for everyone) or do you mean secondary modern (who only take those that didn't pass the 11+) ? Full grammar counties don't have comprehensive schools.
I've seen it from the other side whereby the DC did pass the 11+ and went to grammar school. They felt that they were the stupidest person on the planet, started school refusing, fell in with the wrong crowd, ended up in a pupil referral unit and only 1 GCSE (a D in English). They are fine now 25 years later but put their parents through hell during the teenage years. Had they not passed and gone to the secondary modern, I think the outcome would have been much more positive.
Your op is unclear whether your DD took the 11+ and failed, in which case grammar
was never an option regardless of what you wanted. Or was she not entered at all due to advice from the head.
Hindsight is a wonderful thing, it might not have been a ball if she had gone to the grammar either, bullying happens there too. It sounds like the problem was with the lack of action on bullying at the school she did go to rather than not getting in to the grammar.
If she can work hard from now on, there is nothing stopping her being successful in her life. Don't dwell on the past, that can't be changed, but move on and help her see what she can achieve if she wants to.