One technique when presented with any type of negative feedback, critique, or criticism is to ask for more information, using gently probing questions... such as:
- would you give me some examples?
- under what circumstances do you see this behavior?
- how frequently do you see this behavior?
Listen attentively. And take notes. Thank them for sharing their feedback.

Then ask, is there anything else?

When you have received confirmation they have exhausted their arsenal (and not before), then make one simple statement which pleasantly counters the negative... such as, "That's interesting, because at home he is intellectually curious and highly motivated. I wonder what it could be about the classroom learning environment which is causing you to see him differently." Notice that you refer to them seeing him differently... not to him behaving differently.

Again, you are wise to say this only after receiving confirmation that they have told you every bit of negative feedback that they have... in order to avoid going back-and-forth in countering each other's statements... which tends to escalate into argument rather than focus on working together for the benefit of the child.

Then carefully observe the reaction/reply/response. After hearing your statement, if they have more negatives to share, once again pleasantly ask for examples, circumstances, frequency. Listen well. Take notes. Thank them. Ask them if there is anything more to share. Then pleasantly repeat your statement.

Now that the meeting is in the past, you could still engage in this type of conversation for clarification. For example, you could start the process by writing a pleasant e-mail as a follow-up to your meeting, thanking her for the feedback, saying you'd like to explore that feedback further, and asking your gently probing questions.

Some background information:
1) Kids who've been denied an appropriately paced and challenging curriculum will begin to coast... their brains change as a result of the lack of stimulation... they may become underachievers.
2) Kids may respond to messages they receive by becoming afraid to make a mistake, to ask a question which reveals they don't know something, etc. They may procrastinate, become anxious... and become underachievers.
3) It is also possible that there is no problem... but that the school is engaged in a research study and is evaluating what occurs when various messages are given to various groups of kids and/or their parents.

Stay alert. Be pleasant. If you do not observe a lack of motivation in your child, my advice would be not to share the teacher's message with your child. At least not until you've
1) read up on underachievement,
2) had "the talk" to gather more information from the teacher,
3) encouraged your child to talk with you about his daily observations on experiences and interactions in the classroom.