Hi Sam,
Ask the teacher for a brief written summary of the day - each day your son goes. Keep this up until you are satisfied things are improving - which they should if handled properly.
Using a sticker chart could work well if the teacher is willing to tell you in her note that your son has earned one and if everyone reminds him of the need to behave in order to earn a sticker. Whatever he earns should not be extravagant or easily earned.
This issue is primarily up to the teacher and staff members to handle because it occurs at school. That being said, you have an important role to play in verifying you agree with and in supporting their discipline approach. If your son will not sit in "time out", they should send him to the director's office for a talk - or something comparable. The staff members should be willing to call you to come if necessary. You and the staff can let him know if Mommy has to come, he will be in even more trouble. If you have to go, you need to be firm with him in the way you interact. If things don't improve soon, each of your visits may need to result in a loss of a priveledge at home later that day. The point would be to tell him this when you go to the school and remind and enforce the loss later in the day.
This is not a crisis, all kids go through rough spots. You, your son and your school's staff can easily deal with this challenge.
Good luck!
J. B
Parent Coach