i've no experience of this and i'm not sure what the Service awards requires, but i'm guessing they need to perform a community service. if so...
they could hold an open day for the school to show the local community how it differs from a 'normal' school. most people won't have visited a special needs school. the pupils could help conduct the tours.
would art count? a charity project i worked on before involved schools and a special needs one made models of London landmarks along the Thames. the pupils could do a similar thing for local landmarks. have a word with the local library, or a shop to see if you could get display space.
contact the local council and see if they have any projects that would be beneficial to both sides. helping in a local park would be an obvious one as it would give the council the special needs perspective - often that is overlooked for more 'obvious' disabilities such as wheelchair access, etc. special needs pupils tend to need more tactile playthings - as well as simple signs, etc.
arrange a class visit to another local school - giving those children a chance to see how a special needs school differs and the chance to mix.
erm... that's all i can think of