If they've been showing a genuine lack of interest and potential throughout the year, then I think it's fair to kick them out. However, if they've been performing well throughout the year and are enthusiastic and committed, they should be let back on.
I think if the school has admitted them into the sixth form then that should be for the full two years. It could be hard to enter other schools in year 13 after being removed. It's fine to be selective when selecting students for entry into the sixth form but halfway through, that just causes unnecessary stress for the student and will probably worsen their grades further.
Sure but they lose bragging rights if their kids get AAA like a battery after the school kicked out kids with anything less at the end of the first year.
Of course it's fair. Why should the school use their resources on complete no-hopers?