Unfair marks in swimming?

In swimming lessons, my brother had to swim 500 meters at a time. He was given a grade based on his speed, but the conditions were apparently really unfair. There were 20-25 people in each lane at the same time. Overtaking was allowed but almost impossible (because people were coming the other way). So, you had no chance to swim at your own pace. You were always being slowed down or slowing down others. What should you do?