I'm not sure that the term 'coasting school' has any meaning.
However there is an awful lot of stats published on the dcsf league tables which tell you lots.
for example it tells you how low attainers, middle attainers and high attainers do.
It also gives you the CVA for the school.
From what you describe I would expect the high attainers to be doing badly and the CVA to be well below 100.
If those are true then it sounds like there's a problem.
If those aren't true, then either the school is very good at manipulating figures, or what you think is happening isn't actually happening.....