Hi Happypiglet:
First - you're not being unreasonable. You are sending your child to school to learn. Taxpayers are paying the salaries of staff in school to educate the next generation. It's not unreasonable to expect teachers to be trained to deal with keeping order/ discipline in the classroom. Endless disruptions hold back learning and result in poorer progress than is expected or necessary.
I would approach it positively.
Your DC loves school. He is really interested in learning new things and likes his teacher(s).
However, you are growing increasingly concerned to learn that lessons are being abandoned, that your DC is regularly being sent to get him/ her (the Head) to discipline pupils, etc....
You don't want to complain, but you would like to understand what steps the school is taking to support this teacher and give her additional training to deal with unruly or disruptive pupils.
You would also like to understand whether it is possible (if this is not a single form school) for you to request that your child to be moved away from these disruptive pupils in future years.
If you are suspicious (apologies but I'm an old cynic) and feel this may ultimately result in your complaining to the governors or LEA - I'd ask for minutes on this meeting or acknowledgement that you two have discussed this issue and confirmation from the Head that she/ he will be monitoring this situation. I would also request an update in six weeks (by meeting or letter) from the Head to demonstrate what steps have been taken to mediate against further disruption in class and loss of learning opportunities for all children.
Finally every teacher has their strengths and weaknesses - and each class is different - this may be a boisterous class that an otherwise good teacher with a strong record for performance is seriously struggling to get under control. However, this is 2nd term now, and a lot of time has passed by with no obvious improvement, so it is important that something is done swiftly to support this teacher and make the learning environment more orderly.