Goodbye,
OK, let's spell this out carefully.
Say there are 10 schools, currently all comprehensive. All offer a full range of A-level choices, because they have sufficient pupils wanting to take the full range of options, as well as more vocational qualifications. All have approximately equal numbers of SEN and PP children, all at the average for the area. Teachers in all schools get to teach the full range of ability, and thus - quality of department and school leadership apart - all schools are 'equally desirable / have equal status' as places to work.
One converts to a grammar, taking the top 10% of every other school. It is perceived by parents as 'better', because it gets better raw results AND it will have virtually no SEN children and perhaps 20-25% fewer pupil premium children than all the other schools. The proportion of SEN pupils and PP pupils in all the other 9 tgherefore increases.
The numbers in each of the remaining schools wanting to take 'minority academic A-level subjects' - Further Maths, MFL - fall below the economic threshold, and so cease to be available in those schools. Teachers who want to teach their subject right the way through to A-level - often those with the best subject knowledge - therefore leave. The grammar is obviously going to be an easier place to teach - no children of low or middle ability, no SEN, virtually all from supportive homes - whereas the other schools will be harder, so teachers will gravitate towards the grammar and recruiting and retaining teachers for the other schools will be more difficult.
That's before you even get on to the fact that there will be children in 'the wrong school for them', because the test is so flawed, and the psychological effect on those who have failed on a high stakes test at 10 ...