If the problem is serious enough to need discussion with the teacher, then PLEASE make an appointment! Give the teacher a clear idea of what you wish to discuss at this appointment. Many teachers would agree there is nothing worse than being accosted, however nicely, at the school gate or in a restaurant or bar. Any problem will be better resolved if BOTH parent and teacher are prepared and in the right frame of mind. Remember teachers do have lives!
Assuming you have an appointment, prepare by trying to analyze the specifics, as you and your child see it. The vaguer you are, the harder it will be for the teacher to understand how assistance can be given. The more precise you are, the less likely it is that you will be fobbed off with platitudes or generalized comments, too.
Try to remember that teachers and parents are on the same side. If you have a low flash-point, bear in mind that no teacher’s contract includes a duty to sit and be verbally abused or threatened. It could be a short meeting!
If you remain dissatisfied at the end of your meeting, say so politely and calmly and explain why. You can refer the matter to a more senior member of the school staff and it would be a courtesy to let the teacher know if you intend to do this.