If all studies were based on such small sample sizes as this, then we'd all be in a world of hurt. I'm very surprised it got approved for publishing.
If the difference between means is large compared to the standard deviation of the population under study, you need less n.
Another fun fact: All groups are different given large enough n.
Edit: I'm not endorsing the study - just pointing out that n should not be the most critical thing you look for when evaluating a study. The study is fine as a simple quasi-experiment, testing the hypothesis by induction. I don't see anyone claiming causal links here. Now a study testing the hypothesis deductively needs to be designed.
I don't suspect any school below a rank of 15 would be taken seriously.