My suggestion is this:
In a few months time you are going to be working in a nursing profession and possibly nights again but maybe days. Start keeping the kids at home more often, not cold turkey take away from grandparents home, and start having more time with them at home one on one if possible but also time together as a family. Cut back on you work hours. Surely you don't have to work 5 days a week nights and can get financial aid to cover the extra expenses the income would cover.
Not saying I haven't been there, my degree was a BA in Sociology with a minor in Social Work and a second BA in Psychology and I never worked a day during school, I had financial aid and took full advantage of it so I could take care of my daughter as a single parent, she did eventually go live with my ex and his wife, which was good so I could focus on school, she was uncontrollable and it was really as good thing for her to go to them....
But my point is that school is a full time job and your family is doing without you. Consider cutting back a bit, not saying quit, just cut back, maybe one more night a week off or transfer to days and still cut back to 3-4 days a week then school on the other 2.
It will be a gradual transfer of attachment. In those kids minds grandma and grandpa are mom and dad. It's normal for the primary caregiver to have that place and it's in your letter they are basically raising your kids.
It will be best for the kids and grandma and grandpa to wean them away gradually. They will also have stuff that you'll have to deal with like "that's not allowed at grandma's or grandma lets us do that" you can't be the bad guy and change everything all at once. Just do it gradually, very, very slowly and it will eventually even out and they will feel like you are mom and dad again soon.