I have always opened the conversation with the teacher relating what my child is saying at home. I've opened conversations with teachers with statements like,
"DS says he doesn't do math when you say it's math time"
following up with: "I know that's not the case, can you tell me why he might be perceiving this to not be math?" (followed with, perhaps DS doesn't recognize this as math)
or
"DD has been saying several times that she wishes she didn't have to go to school. She wants to go if she could learn something, but she keeps saying that she'd rather stay home and play because she doesn't learn anything"
followed with "of course, we know she's there for the lessons, why might she perceive she's not learning anything?"
These start the conversation. From there, the conversation must get more involved, of course, to put in place appropriate differentiation or accelerations. I have found a lot of success, however, with starting off with my child's words to illustrate how the pace of instruction if affecting my child.
