AS well as crafting myself, my DD has Down Syndrome so I have some knowledge of issues that the teenagers may encounter but obviously the range of abilities may vary enormously.
Have you any idea of what disabilities your group have? Are you taking learning disabilities and/or physical disabilities.
Here are some idea off the top of my head, if you need any further help, please just shout and I will try to help as much as I can.
Cardmaking- basic card blanks, glitter glues, die cut shapes, even some easier stamped images depending on capabilities.
Cross stitch on something like the larger gauge plastic canvas and a thicker tapestry type thread as opposed to embroidery silks.
AS Whizzz said, threading larger beads onto cord- would be great for fine motor skills.
Fabric painting, use sponges, hands, feet, potatoes....
Playdough can be very theraputic and again, great for building up hand muscles, coordination, control, fine motor skill...
even rolling out sausage shapes and coiling them up into pots. I have never ventured into other moulding products- my daughter is 6- but I am sure others can advise- but be aware there may be sensory issues involved.
I will keep thinking......