M.Y.
I am a preschool teacher and am surprised how each of the schools handled the problem. We try to call the parent only as a last resort, (maybe it was a last resort??)as then the child may learn that once he has a tantrum they will call mommy or daddy to come pick me up. I have no ides if this is what is happening in your child's mind.
Is the school trying to work with you on this? Is your child able to verbalize his feelings at all? It would be good to find out what usually causes the tantrums. Hopefully you have a caring teacher who has your child's best interests in mind and will work together with you to resolve the problem.
Meanwhile, the best you can do for your child is to let him know you care.Try to get him to verbalize his feelings to give you some clues. He may need a little extra attention and hugs right now. Try to keep an upbeat and positive attitude about his school and watch what you say in front of him. Children pick up on their parent's feelings very quickly.
If he is strong willed it may be difficult to break this "pattern" of behavior. It is important to find a teacher who will not get into power plays with him,but try to get to the root of the problem. Hopefully it is a "phase" and this will resolve on it's own. I will keep you in my prayers.
M.