I had a music and an academic scholarship about a hundred years ago (ok, about 20 years ago). I now have DCs approaching secondary school age so have looked at several schools' scholarships and bursaries and would second the view expressed by another poster that schools now give far more away in bursaries than as scholarships.
On the academic scholarship front, the maximim award is usually 20% and the awards usually range between 5-20% remission of fees. Music scholarships normally just entitle the child to free music tuition (2 instruments normally), though some schools offer a small remission of fees as well.
You can apply for a bursary in addition to any scholarship award. Bursaries are means tested and as far as I have gathered, the cut-off point is a household income of around £50k. If you get a bursary, the school will means-test you every year to ensure you still qualify for the award.
In my day, having a muisic scholarship meant that you were expected to participate fully in the school's musical activities - play in every concert, sing in choir, etc. Quite a bit of practice was also expected but no more than any child keen on music would do naturally anyway (I was already practising 2 hours a day voluntarily by the time I was 11).
I suppose an academic scholarship MIGHT be withdrawn if a child really lost the plot, but this never happened to anyone I knew and I haven't heard of it happening to children of friends, etc.