I have been on SSDI since 2003. I don't have a definitive answer to the question, but I can tell you my experience so far.

I began attending classes at the local Jr College. In CA, being on SSDI (or other assistance) qualifies a person for a grant that pays tuition only at Jr/Community Colleges. While I was attending I got a claim review form asking me a handful of questions about my condition...one of those was "have you begun school or attended any training programs"...I answered honestly and got a letter back saying there was no need to review my claim at this time. I graduated with an AA in May 2008...heard nothing from SSDI. I have been attending a 4-year University since then and have gotten the review form again ( a few months ago)...exact repeat of last time. I Graduate with my BA in December and have begun applying to graduate schools...not a word from SSDI.

I think that part of the issue between attending school and working is that they are not in any way the same! Yes, I have to attend classes at specific times, however, those times are at most 3 hours (for lab classes) but usually 1-2 hours, and they are not every day. Work is every day, at specific, and usually longer hours. Also, if I don't finish a project/paper/etc by a due date, I can often discuss the problem with a Professor and get help/extended due dates/etc...not so much with a job!

One thing to keep in mind is that I do go through disability services for students. They are required to provide me reasonable (and specific) accommodations...two of mine are consideration for due date changes and absences...I've actually only used the due date one once, and the professor actually changes the project completely but not the due date.

Anyway...my point is that I have not had any trouble with SSDI and attending school...and I have known other people doing the same. Ability to succeed in school is not the same as ability to succeed in a job...and SSI/SSDI knows this.