Can I stay on my parent's health insurance if I go to graduate school and am over 26?

No. Aca allows children to receive health insurance coverage through age 26. I don't think that you can remain on their policy, even though you will be in grad school. Check with the insurer.Health insurance is important; if you are unable to afford a standard policy, i would purchase a high deductable 'major medical' policy, which would provide coverage for a catastrophic conditions.