expat, to answer your question at least partly. 10 out of the 11 kids who applied got into Oxford or Cambridge this year which is a pretty good success rate. We have a very good programme of support which begins with screening and encouraging students with mainly A*-B at GCSE who are continuing to show talent and commitment at AS level to apply and then supporting them in weekly sessions which involves everything from stretching them in their particular subject, helping them with UCAS forms, preparing them for interview etc, etc. This is in addition to subject specific weekly extension lessons which are run in each department for A or B students who are aiming to pursue that subject at university but Oxbridge students are likely to do both as well as a whole load of other extra curricular activities. We also teach the Extended Project and again Oxbridge students often do this. BTW, most schools offer Oxbridge support even if they don't have the resources to offer the amount we do and schools in our area work together with us to provide trips to Oxford, talks from Oxbridge recruiters etc.
I'm not involved in this personally so I could tell you how many students were originally approached to apply to Oxbridge but I'm fairly sure it would have been significantly more than actually did. There are a number of reasons for this of which the main ones are probably as follows:
1.) Oxbridge courses are quite traditional and not to everyone's taste. Students often find a course or combination of courses (which Oxbridge often don't offer) which more closely matches their interests elsewhere.
2.) Many of our students choose to stay close to home for cultural and/or financial reasons. BTW we have 2 Russell Group universities 20 minutes away with good public transport and several more universities very near.
As for your question about fractions of time, this is actually quite a strange way of looking at teaching. Maybe this would be more relevant to a secondary school teacher (which I have been but am not at the moment). At 6th form level in my subject we don't really break down time and attention into different ability groups. I can't really see how you could. Just as we have particualr study sessions for able students, we have study support sessions for students who struggle in each subject so if a student wasn't keeping up because of absence or because they were finding something difficult, I could book them in for support. Again, this is common in 6th form colleges. Or I could arrange for them to come and see me or email me outside lesson time so I don't think I spend a disproportionate amount of time with students at the bottom of the scale.
I think at 6th form it's quite easy to let bright students dominate in fact esp since so much of lessons are discussion based so to avoid that I spread out questions and often get students to work on something in pairs or groups before going around and asking for a response from every student.
Most of the tailored feedback which really leads to progress comes from marking where I might write up to half a side of notes at the end of essay of strenghts and targets for improvment as well as marginal annotation. There are probably some bright students who end up getting more feedback because they are more likely to meet deadlines and those students who want to do well are more likely to seek advice and redraft and redraft coursework.
In all, I'd like to think that my time and attention is divided equally between all of my students. This is much easier to do in 6th form where there are few discipline problems and where students have made an active choice to be but, if anything, I'd say the most able are more likely to get more attention because they're so much more likely to participate in discussion and engage with their teachers as part of their interest in the subject and desire to improve and meet the entry requirements for university.
Thinking back to my time in secondary school, that probably tallies too although of course a lot of attention went to kids who were disruptive or silly and that attnention was actually not about academic achievement at all IYSWIM.
As I've said I've got really clear memories of my brightest students dating back over 10 years because their writing and comments were more likely to be original and interesting and I would have more interesting things to say to them in response rather than pointing to spelling mistakes or whatever which might be the focus for a very weak student. I can actualyl remember individual pieces of work from bright students from years ago and I can't really say that about any other studednts. These were the students who would stay behind at the end of the lesson just to chat or I'd have a chat to them as I'd give back work or whatever.