I am speaking here as a Head of History in a large comprehensive school.
The problem with History setting is that in the vast majority of schools is that Options are 'blocked'. This basically means that all pupils in each 'block' (we have 3) who have chosen History will be in the same group, unless there are enough to have 2 groups. We currently have 5 groups in yr 10 and 11. 4 we have managed to set but 1 is mixed ability. Every year there is at least one if not 2 mixed ability groups.
It can be disconcerting and worrying for pupils and parents who are used to being in top sets for everything to find themselves in a class with less academic pupils and this worry is natural. I have answered many parental queries on this subject in my time.
As has already been said, we are used to teaching mixed ability and have been trained to do so. It is not ideal, I grant you that, but it is perfectly possible and can be done very well. Sometimes it can actually improve performance, particularly from those who may be C/D borderline. They 'work up' if you see what I mean.
To give you an idea of how it is done, I currently teach a class with targets of everything from A*- F. They are all taught the same subject content but to differing depths. I have them sat in groups according to ability some of the time so brightest together, weakest together with TA, middle togethet, although quite often I will mix the middle up with the bright.
Each lesson they will have slightly different tasks/success criteria according to their VA. They choose which task they do. For example if we are evaluating sources, the weakest will be asked to describe what they see, the middle to explain and the brightest to evaluate for utility and interpretation. Same task different expectations.
The key to success is good, clear differentiation as explained above. I also use questions which are labelled according to grade and pupils can choose which questions to answer.
A good History department is used to this type of situation and will teach accordingly.
I must also say, as the topic has been mentioned, that I would never discourage a child from choosing History because they are not 'academic'. That is wrong in my opinion. If a pupil has a keen interest in the subject they should be encouraged to choose it and supported all the way. I am a firm believer that history is, and should be, for all.
I attend local Heads of History meetings. And some schools boast of stunning 90% results but they don't say that they only allow pupils to take the subject if predicted and A or a B. it makes me very angry to see that attitude.
Speak to the History Department and you will be reassured.