In order for some kids to function in the mainstream of schools, they need special attention. The paraprofessional is like a dedicated attendant that accompanies the student to his or her classes in order to be on hand if something should come up. The paraprofessional is an employee of the school and works with the child from the time they get off the bus in the morning until the time they get on the bus in the afternoon. If for some reason the paraprofessional is absent, they get a substitute.
That's not necessarily so. Sometimes one or more paras are assigned to a classroom, not a particular child, and in the event the para or paras don't show up, it's just too bad for the teacher. She might have to deal with multiple developmental/behavioral problems up to including violence or diaper changing all by herself.
I'm sure it varies by school and district, but that is the reality of SPED teachers.
Again, it depends on the State and District requirements.