AndieWalsh, I agree with what you're saying and I understand your outrage as I was in the same situation myself. It is exclusion on the grounds of disability and if it is a question of staffing, then the school should be using these types of arguments to support statementing application.
For a DDA claim, you have to prove (a) disability and (b) discrimination e.g. less favourable treatment.
However, on my reading (emphasise MY reading!) this can be complicated. The case of the London Borough of Lewisham v Malcolm [2008] UKHL 43 (which is applied in the education context by the Court of Appeal case R (N) v London Borough of Dagenham and Barking Independent Appeal Panel (Feb 2009)) says that in deciding whether the disabled person had been treated less favourably, the court must compare his/her treatment with that of any other person who acts in the same way. If the treatment was the same, there is no discrimination.
This means that the law says the question you should ask is not whether school excldued a child from a trip because of disability but whether a school would exclude ANY child who runs away and presents a risk on a school trip irrespective of disability. If they would, irrespective of whether this is bad practice, it is not discrimination.
However, thre school do have a duty to make reasonable adjustments to accomodate disabled children and they could be in breach of that if what they are being asked to do is considered reasonable in terms of staffing etc (remember the DDA EXCLUDES provision which should more properly be provdied via SEN provision e.g. auxillary aids)
I understand that the Equality Act (just received assent last month but not yet in force) has made changes so that a new ground of disability discrimination, called ?discrimination arising from disability? is created and that this sidesteps the effects of Malcolm. It also introduces indirect disability discrimination. But this is not yet in force.
Here, the school is going to see if they can resolve this. I think Awaywiththefairies needs to see what the outcome is while reminding them of their inclusion duties.