title: Count maximum value in each group with Pandas
body: <p>Suppose I have a dataframe <code>df</code> looking like</p>
  <pre><code>    school   score   student_id
  0    1        100.0   965
  1    2        64.0    1483
  2    2        100.0   1055
  3    2        68.0    1806
  4    1        100.0    971
  </code></pre>
  <p>I want to find how many maximum scores in each group and get something like this:</p>
  <pre><code>school count_max
    1       2
    2       1
  </code></pre>
  <p>How can I do it?</p>