ON the face of it, the proportion of pupils who get five or more Highers in S5 - the benchmark commonly used for league tables - is a good measure of how well a school performs.

And it can be. It is undoubtedly true, as inspection reports have shown, that some schools which appear at the top of The Herald's league tables are among the finest in Scotland.

However, there are many schools which boast some of the country's best inspection reports, who are also getting all their brightest pupils five or more Highers, who will never feature at the top of the tables because of the way they are calculated.

The proportion of pupils passing five or more Highers is not worked out, as one would expect, from those who actually sat Highers in S5. Rather, it is a percentage of the total number of pupils a particular school had in S4, regardless of their academic ability.

In other words, schools are being judged more on the make-up of their pupil cohort, over which they have no control, than on how successful they are at getting bright pupils through their Highers.

Because middle-class schools - and those in the private sector for that matter - have greater proportions of pupils staying on to sit Highers they will almost always appear to be better performing.

That may even lead to the perverse situation where a school which is actually failing some of its pupils would still appear higher in a league table than one that is not, but has fewer pupils in S4 capable of sitting five Highers.

Brian Boyd, emeritus professor of education at Strathclyde University, believes parents should look at a great deal more than league tables when choosing a school.

He said: "Parents are often not particularly well-informed about a school, so they tend to go for exam passes as a proxy because that is what is required to get into university and, unfortunately, that can give the wrong impression about how good the school is.

"What also gets lost... is whether the school is good at serving the whole of its pupil community."