I love my slow cooker and I do leave it on while I am at work without any issues. I don't think after school is long enough, most meats you use in casseroles are tougher cuts which need alot of cooking to make them tender.
I also do corned beef in mine and it's so tender!
Briggy's suggestion of borrowing one from a friend for a while is a great idea - this is the perfect time of year coming up to try one out!