Oh yeahh one other thing. Apart from the whole issue of space, my old grammar school would only let you take 3 AS levels if you got less than 5 A-A*s. That was the benchmark for taking the commonplace 4, though that seemed like a dumb rule to me
Also OP, found this for Ilford county grammar sixth form: (form the prospectus on the website)
Oversubscription Criteria
It should be pointed out that, such is the number of
applications we receive, it is not always possible to invite all
those who meet the minimum entry criteria to the school
to discuss admission. Subjects such as Biology, Chemistry,
Economics, Mathematics and Physics are in great demand,
with an initial application to place ratio of around 5:1.
When this occurs, then our oversubscription criteria are
applied to candidates who have opted for those subjects.
1. Candidates are sifted according to a calculation of their
predicted average points score. In other words, those
with the highest predicted GCSE average point scores
will be invited to visit the school. In 2010 the average
points score was set at 7.1*, but it can sometimes be
higher or lower than this.
2. Anyone who falls below this average, and who wishes
to study oversubscribed subjects, will be eliminated from
consideration at this point. It for this reason that we
suggest you select a reserve subject or subjects so that
we have the maximum flexibility in considering your
application.
3. Following the discussion with a member of the Senior
Leadership Team, places will be offered on the basis of
their suitability for their chosen programme of study.
4. Should two candidates wish to follow identical
oversubscribed courses and offer identical academic
profiles, then a place will be offered to the candidate
who lives closer to the school (in accordance with
London Borough of Redbridge admissions procedures).
Important – Predicted Grades
When applying the academic criteria we must act upon the
information we have been given by the student’s current
school at the time of application. Usually this is predicted
grades or mock grades.
*Grade A