Hi Julie,
Bravo for looking to help this little guy!
First, many children at that age have trouble sitting still. But if you think he might be showing signs of ADHD, perhaps it's time to discuss with his parents the notion of having a discussion with his pediatrician about the problems he's having in pre-school.
Asking a young child with untreated ADHD to sit quietly is like asking a person to stand still when a truck is about to hit him. I personally don't think it's fair to expect him to control himself if he's incapable of doing so. Punishing him by removing him is not something I'd recommend. Until he's evaluated for possible ADHD, I would instead, reward him every time he's able to be quiet; even if it's for 10 seconds. I advocate positive reinforcement rather than punishments, whenever possible.
I would also make sure he's in front of the teacher and if possible, seat the other children just a bit further away from him. Have the teacher engage him by asking him questions, so that his talking is appropriate. Reward him when he controls himself (praise, tokens, etc.).
Fidgets are also great tools to help children- and adults- stay calm. Let him play with silly putty, a "stress" ball, or other tactile object that he can squeeze or manipulate. Some kids listen better when they can doodle on a piece of paper.
Give him some responsibilities while in group time- maybe by helping the teacher. Let him stand up instead of sit down. Or if there's a large ball in the room, let him sit on that so he can move around.
Still, if he does indeed have ADHD and it's not treated, it would be unfair to expect him to exert much self control, especially at such a young age. In which case, I would suggest that you and the teacher lower your expectations until he either matures a bit more or gets treatment (if indicated by a professional).
Hope this helps!
Terry