Yes school and vocational training are not classified as work. Neither is volunteer work. Work is based on income alone.
can you still go attend school or traning if you are on ssi/di
If its a hearing to see whether you are eligible you need representation. If you already have benefits and its a reassessment then you just need a letter from your psychiatrist. Regardless you need substantial evidence from your psychiatrist. Your local independent living center should be able to help:
http://www.ilru.org/html/publications/directory/index.html