Top tier that I know of is roughly, NLCS, SHHS, Highgate, Channing, in that order - open to correction, just my impression.
For grammar, HBS, DAO in Potters Bar, St Michaels, even if you aren't Catholic at sixth form, depending on where you live you could apply for Latymer .
Heard St. Helens is nice, also CLGS and others but maybe further away.
At sixth form the subject is important, so even if she seems not to have high enough marks, its still worth applying to any of these because you don't know what gaps they will have in particular courses.
Do you want single sex or mixed? If she likes "boy" subjects, girls might be better imho.