I just went on the AMDI website and it looks like the eye-blink switch attaches with a head strap, so I don't think the student would have to hold their head very still. We have a twitch switch in our lending library but I don't believe we have an eye-blink switch. They're not one of the more frequently used.
The AMDI eye blink switch is the high tech version (
http://www.amdi.net/store/product.asp?intProdID=303), around $625.00, but there's a low tech version for $125.00 from Enabling Devices. (
http://enablingdevices.com/catalog/capability_switches/hand-finger-body-switches/eye-blink-switch)