How about sewing some long, skinny pockets into the borders of the quilt? You could fasten them shut with velcro or zippers. You could then put weights in the pockets to give adequate weight but they could be removed when the quilt needs to be washed. Sort of like the weighted vests that some autistic kids use. Hope I explained that ok. Check with your occupational therapist at the school to get a look at how the weights in a weighted vest are constructed for a visual.