it can be perplexing when a child exhibits different behaviors at school than he does at home, but it is not at all uncommon. Home, especially for an only child, is a place where children feel most comfortable. At home, your child has adapted to you and you to him. Each family is its own little system with its own culture that your child knows best. While I understand that you are not overly permissive, I am sure he does get lots of attention, affection, and support at home to an extent that teachers can never replicate.
Adapting to school can be a tough process for a little one. School is a place where he has to cope with lots of extra noise and stimulation. He has to manage the stress of his relationship with not only the teacher but all the other children around him. He also has to contend with different rules and expectations, such as having to sit still for activities like circle time or seat work. He may have to make transitions according to the school schedule (when to play, when to sleep) that do not feel comfortable. For many little children, these characteristics of school make it a very stressful place to be.
You are correct that a tantrum of over 20 minutes is long for his age, and are probably a sign that he is really loosing control (as opposed to a deliberate 'performance' tantrum done to achieve a particular end, such as getting an extra dessert). There is most likely more of a pattern to his outbursts than the teacher has identified. Tantrums in young children are most often related to some physical issue (low blood sugar, fatigue, earache etc) or anxiety. If you can rule out the physical issues, then there is probably something about school that he is finding overwhelming. It could be jealousy, feeling left out, anger at having to share--so many things. He may find it too hard to share his teacher with his peers, to share toys, or to meet expectations for hours upon end. He may be anxious about the discipline used at school--for example, if he finds himself put in time out on a daily basis. It would be great if the teacher could look more closely for a pattern and the principal could do an observation to find out what seems to be triggering the tantrums.
I recommend you seek help from a clinician with expertise in young children. School problems are one of the most popular topics we deal with among children under 5. A psychologist, social worker, or licensed counselor can all assist you in helping your son learn to cope with strong feelings and teach him better ways of handling his emotions. Ask your pediatrician for a referral to someone who is willing to work with the school to help your son. With a good clinician, you should see improvement in a few weeks.
In the meantime, I think many tantrums can be avoided (easier said than done, I know). Once the tantrum starts, its hard to get the child calmed down, so best to be as proactive as possible. Once you have learned more about what situations are most likely to trigger tantrums, you can focus on those trouble spots. It works very well with children his age to reward desirable behavior instead of punishing. Kids this age are much better at doing something than they are stopping themselves from doing something. Best to give active directions like "Use a quiet voice" instead of "Stop screaming!" Another good way of preventing power struggles is to encourage the child to do something he can not do at the same time as the unpleasant behavior. For example, you can tell a child to 'use a quiet voice' because he can not scream and talk quietly at the same time. Teachers and parents can help a lot by making sure the child gets the most attention for doing things you want him to do--even just sitting quietly can be worthy of praise, hugs and a reward.
Though time out is quite popular, but it is far from perfect. Time out is often over used, and if he is in time out more than twice daily than clearly it is not an effective form of discipline for him. Moving a child into time out when he is out of control is often a precursor for the child lashing out physically, and it can easily become a physical battle between adult and child (which should only be a very last resort in an extreme situation like a child running into a street). Instead of putting him in time out, ask the teachers to take something away from him (example "If you do not put the toy down, I am going to take it away. You decide."). If there is not a thing to take, teachers can take themselves and their attention away (example "I do not like screaming. You can talk to me again when you can use a quiet voice).
You might also check out two great books: The Kazdin Method by Dr. Alan Kazdin (very good book on positive discipline, and information about how you can use time out effectively if you choose to do so), How to Talk so Kids Will Listen and Listen so Kids will Talk by Adele Faber and Elaine Mazlish (there is also a teacher version)
Disclaimer: This post is for informatinal pursposes only. It is never intended to replace face-to-face medical and psychological care. This post is not intended to create a patient-clinician relationship, nor to give or rule-out a diagnosis.
When I said he does not get his way, I meant he does not get everything he wants when he wants it. So we do not think it is something he expects to happen at school.