He is even worried pupils who get Government grants to come to school are not cutting back their hours of employment.

His comments are made in the latest issue of the school magazine. Mr Haggarty is talking specifically about his experiences at St John’s, but it is thought the concerns he voices can be applied to other schools, locally and nationally.

He writes, “The guidance team recently reviewed for me the results of S5/6 prelims, which were disappointing for many pupils.

“I spoke to the fifth and sixth years about being more actively organised in their study.

“Above all, I emphasised they could bring about an improvement in their marks if they realised that, apart from a few days of holiday, they should really commit the bulk of the Easter break to solid study.

“They responded with a smile when I told them it should be no comfort to simply say that their pals had not done any better than they had.”

Mr Haggarty said most pupils would not want to see their exam certificates bearing marks similar to those achieved in the prelims. He urged them do something about it.

He went on, “What really struck the guidance team were the very high hours of paid employment which our seniors are involved in.

“Twenty to 25 hours is not at all uncommon.

“We do not believe that there will be much time left for quality study after a pupil has been at school for 27-and-a-half hours, worked for 20 to 25 hours, taken their meals and socialised with friends — spending their hard-earned money — before giving any time to study, which is a very low priority.

“I am very concerned pupils who are paid an educational maintenance allowance (EMA) to encourage them to focus on their studies have not cut back on their working hours.”

Last week Deputy First Minister Jim Wallace was in Dundee to announce the national roll-out of the EMA programme, which was piloted in the city.

It involves pupils who have reached the leaving age of 16 being paid up to £30 a week to stay on at school in the hope of gaining more qualifications and eventually finding it easier to move on to further or higher education or into work.

Teenagers whose family income is up to £30,000 a year are eligible for an allowance.

However, the Executive has promoted the programme as being aimed mainly at pupils from low-income households — the pupils most likely to need part-time jobs to help their families make ends meet.

In his newsletter, Mr Haggarty explains St John’s is contributing to a study on the effects of work on pupil attainment being run by Edinburgh University.

Surveys based on national exam results from two years had been “staggering”, he wrote.

Pupils working 10 to 15 hours a week would, on average, score 55% to 60% on tests, with no guarantee of an A or B grade.

Those working for 15 to 20 hours would either scrape a pass or just fail. Those working more than 20 hours a week would typically get only 40% or below — a clear failure.

Mr Haggarty pointed out Dundee University recommended to under-graduates they work no more than 10 to 12 hours a week.

He concluded, “I think the message is very clear and I would appeal to parents to be much more concerned about the hours that our younger people work part-time.

“The money they earn is soon spent. The benefits of their studies impact on their opportunities for the rest of their lives.”

A Dundee City Council spokesman said, “Schools regularly put out newsletters to parents drawing attention to issues that may affect their child’s education.

“Although there is no objection to pupils working part-time, this must be balanced against time for their studies.

“The school is always interested in pupils performing to the best of their abilities and the comments in the newsletter reflected this.”

St John’s, in Harefield Road, is the only one of the city’s three Catholic secondaries with a long-term future.

The others, St Saviour’s and Lawside, are to be scrapped and replaced by a building on a new site as part of the council’s £80 million schools revamp.

HM schools inspectors visited St John’s in 2001 and gave it a glowing report, with four quality indicators judged very good, another 14 good, only two fair and none unsatisfactory.

Some 79% of lessons were said to be good or very good.