Boy, it varies.
My younger is in the autie room 1/2 time and mainstream halftime. He says the aides in the autie room help students with homework, work with a small group when the teacher is busy with other things, and play games with students who have earned them as rewards. When they are in mainstream, the aide will sit next to some students who might need help or reminders to stay on task. If none of the autie/aspie students in the mainstream need that level of support, they sit in the back of the room ususally taking notes for the students to use later or jutting down the homework assignement. Although it changes based on what everyone had happening, the aides are more or less assigned certain students.
My older child is in EBD 1/2 days and mainstream halftime. The EBD aides are more assigned to the program then to a child. They stay in the EBD room unless called down to the mainstream classroom to remove a child. They watch to make sure the children are on task. They can be responsible for keeping in contact with mainstream teachers to make sure homework is handed in.
My older son has a friend who is servely physically handicapped. The friend has an aide assigned to him full time. The aide is responsable for pushing his wheelchair to classes, taking notes for him, feeding him, and taking care of bathroom breaks.
I know very little about the aides for the mentally ret*d group. I've seen them move with the children to the lunch room or gym or watch plays and stuff with them but that's all I know about that group.
The LD aides I know of are in the mainstream classroom. They take notes and scripe for LD students.
So, it varies. I hope that helps.
BeeBee
Edited to add about the LD aides....forgot them the first time.