SSI is basically designed to help ppl who are disabled but wish to hopefully re-enter the workforce. no you don't have to pay back monies you've received. going to school is a positive approach to possibly be employed in the future to enable a job skill. no money will be withheld while you go to school. if you decide you want eventually to try to work the only money that will be detracted from your check is one dollar for every two you make. you can work up to 20 hours/wk.to see how it goes. if you find this isn't successful with your disability you quit work and go back to your full SSI benefit.
vocational rehabilitation a federally funded agency run by the state you live in may help you get funding for school, counseling and they have a job bank to send you out for interviews to help you get a job. wonderful agency, FYI. it too is designed to help disabled ppl try to get back into the workforce.
hope this helps.