Assuming that you are a minor student and you are talking about disability income from Social Security (SS), SS disability income received by a minor is considered income for the parent and reported on FAFSA parent schedule A (line 84). The SS disability income is reported under the minor’s SS number and may have to be reported on the minor’s 1040 however.
If you are a parent who has to include parental info on FAFSA and you’re talking about receiving disability through some insurance policy, the income may or may not be taxable. If it’s taxable, it would be included in you AGI on your 1040 form and reported on FAFSA line 79. If it’s not taxable income then it’s reported on FAFSA schedule B (unreported income) line 85.
Whatever you do, the income only gets entered once on FAFSA, do not report it twice.