The first time you hear the whine, sit down and ask your child what he means. What did the teacher do or say that would make him think this? Did he miss an assignment or say something that might have upset the teacher? Explain that sometimes teachers have a lot of decisions to make and that perhaps your child waded in at the wrong time. Explain what "the straw that broke the camel's back" means and imagine having 26 10-year-olds asking for everything from tissues to an explanation of quantum mechanics while you have to teach a lesson on the solar system.
The second time you hear the whine, try Step 1 again. If the complain persists, call your child's teacher (just to check in) and tell him that you would like to help get things straightened out if possible. The teacher may not even be aware of a problem (remember those 25 other children?) or welcome an opportunity for some grown-up help.
When you meet with the teacher, be honest about what your child has told you and forthright about the steps you have already taken as a parent. Depending on the teacher's response, you may bring your child in at this point to see that you and the teacher are ready to solve the problem.
Allow the teacher and the child to negotiate without interference to see if the problem is going to be solved. This may happen with you sitting there at the table with them or it may happen between teacher and student and a phone call between you and the teacher, depending on your "read" of the situation. If you are beginning to get the idea that your child has a point, stay put at the table.
Follow up with the teacher and your child as to how things are going and be ready to mediate. Both your child and her teacher should now feel comfortable asking for your help instead of thrusting you into the middle of their problem.