The actual comment was 'Because they are the group most likely to leave school having under achieved.' and this is true for nationally recognised academic achievement.
At GCSE, 60% of all children in England achieve 5 A*-C grades whereas just 10% of looked after children (LAC for 12 months or more) do.
However, you must also look at the fact that 28- 33% of those LAC have a statement of Special Educational Needs. That is before looking at the instability of placement factor (children having many moves of care placement), the amount of schooling missed, emotional barriers to learning etc.
Just to dispel a few myths- most LAC do have parents, indeed around 20% of children in care actually live with their own parents, though of course the majority do not.
Looked After Children are not bad children- only around 5% of children in care are there due to their own behaviour, and arguably in many of those cases, their behaviour is a product of their early childhood.
Many looked after children are extremely resilient, and show greater fortitude than adults facing similar circumstances.
All the statistics show is that actually 5 A*-C GCSE is not a very good measure of achievement! Many LAC have actually achieved enormously well in terms of overcoming the barriers to education they face.
They do have priority in admissions, but only to local community schools not to all schools (ie VA, VC, foundation, grammar etc).